Spain is preparing to hold its third parliamentary election in less than four years. But even though the exercise is familiar, uncertainty surrounds the election's outcome.

Polls indicate a substantial share of voters, about one-third, hadn't decided by the final week of campaigning how they would mark their ballots on Sunday.

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In this Tuesday, April 16, 2019 photo a man deposits an empty wine bottle into a recycling container with an election poster showing the face of Socialist prime minister Pedro Sanchez in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. Election message reads 'Make it happen. Vote PSOE, The Spain that you want'. (AP PhotoPaul White)

Spain is preparing to hold its third parliamentary election in less than four years. But even though the exercise is familiar, uncertainty surrounds the election's outcome.

In this Tuesday, April 23, 2019 photo, Spanish election runner Citizens party's Albert Rivera, right, and Prime Minister and Socialist Party candidate Pedro Sanchez on a screen in a studio, prior to the start of the second televised live debate ahead of Sunday's general election in San Sebastian de los Reyes, on the outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years.  (AP PhotoBernat Armangue)

A rising populist party also confounds confident predictions at the other end of the ideological spectrum. The Vox party is expected to give the lower chamber of the Spanish parliament its first lawmakers from the far right since the 1980s.

In this Tuesday, April 16, 2019 photo, the sun reflects on a banner of Popular Party's candidate Pablo Casado in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Tuesday, April 16, 2019 photo, the sun reflects on a banner of Popular Party's candidate Pablo Casado in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Tuesday, April 16, 2019 photo, Popular Party's candidate Pablo Casado, top and center, smiles while posing for snapshots with supporters after an election campaign event in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Tuesday, April 16, 2019 photo, Popular Party's candidate Pablo Casado, top and center, smiles while posing for snapshots with supporters after an election campaign event in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Saturday, April 6, 2019 photo, Spain's far-right Vox Party candidate Santiago Abascal smiles to supporters during a VOX event in Leganes, outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. . (AP PhotoBernat Armangue)

In this Saturday, April 6, 2019 photo, Spain's far-right Vox Party candidate Santiago Abascal smiles to supporters during a VOX event in Leganes, outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. . (AP PhotoBernat Armangue)

In this Tuesday April 23, 2019 photo, the torn remnants of election posters by UnidasPodemos candidate for parliament Alberto Garzon and the right wing Falange party are seen on a wall in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoPaul White)

In this Tuesday April 23, 2019 photo, the torn remnants of election posters by UnidasPodemos candidate for parliament Alberto Garzon and the right wing Falange party are seen on a wall in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoPaul White)

In this Saturday, April 6, 2019 photo, voters smoke a cigarette at the end of an event hosted by Spain's far-right Vox Party in Leganes, on the outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Saturday, April 6, 2019 photo, voters smoke a cigarette at the end of an event hosted by Spain's far-right Vox Party in Leganes, on the outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this April 10, 2019 photo, Vox banners hang from a bar in Brazatortas, on the edge of the Alcudia valley, central Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this April 10, 2019 photo, Vox banners hang from a bar in Brazatortas, on the edge of the Alcudia valley, central Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Monday, April 15, 2019 photo, Spanish retirees wait for the arrival of Prime Minister and Socialist Party candidate Pedro Sanchez at an election campaign event in Leganes, outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Monday, April 15, 2019 photo, Spanish retirees wait for the arrival of Prime Minister and Socialist Party candidate Pedro Sanchez at an election campaign event in Leganes, outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Sunday, March 24, 2019 photo, activists paint an "estelada" or independence flag on a water tank near Barcelona, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoEmilio Morenatti)

In this Sunday, March 24, 2019 photo, activists paint an "estelada" or independence flag on a water tank near Barcelona, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoEmilio Morenatti)

In this Wednesday, April 24, 2019 photo, Podemos party leader Pablo Iglesias, delivers a speech during an electoral meeting in Barcelona, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years.  (AP PhotoEmilio Morenatti)

In this Wednesday, April 24, 2019 photo, Podemos party leader Pablo Iglesias, delivers a speech during an electoral meeting in Barcelona, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoEmilio Morenatti)

In this Wednesday, April 24, 2019 photo, a banner of Spanish election runner Citizens party's Albert Rivera is seen in the background in a subway station, in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Wednesday, April 24, 2019 photo, a banner of Spanish election runner Citizens party's Albert Rivera is seen in the background in a subway station, in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

Socialist Prime Minister Pedro Sánchez is hoping for a strong mandate to stay in the office he assumed 11 months ago when a no-confidence vote ousted his conservative predecessor.

In this Tuesday, April 16, 2019 photo a man deposits an empty wine bottle into a recycling container with an election poster showing the face of Socialist prime minister Pedro Sanchez in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. Election message reads 'Make it happen. Vote PSOE, The Spain that you want'. (AP PhotoPaul White)

In this Tuesday, April 16, 2019 photo a man deposits an empty wine bottle into a recycling container with an election poster showing the face of Socialist prime minister Pedro Sanchez in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. Election message reads 'Make it happen. Vote PSOE, The Spain that you want'. (AP PhotoPaul White)

A rising populist party also confounds confident predictions at the other end of the ideological spectrum. The Vox party is expected to give the lower chamber of the Spanish parliament its first lawmakers from the far right since the 1980s.

Aritz Parra contributed to this report from Madrid.

In this Tuesday, April 23, 2019 photo, Spanish election runner Citizens party's Albert Rivera, right, and Prime Minister and Socialist Party candidate Pedro Sanchez on a screen in a studio, prior to the start of the second televised live debate ahead of Sunday's general election in San Sebastian de los Reyes, on the outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years.  (AP PhotoBernat Armangue)

In this Tuesday, April 23, 2019 photo, Spanish election runner Citizens party's Albert Rivera, right, and Prime Minister and Socialist Party candidate Pedro Sanchez on a screen in a studio, prior to the start of the second televised live debate ahead of Sunday's general election in San Sebastian de los Reyes, on the outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Tuesday, April 16, 2019 photo, the sun reflects on a banner of Popular Party's candidate Pablo Casado in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Tuesday, April 16, 2019 photo, the sun reflects on a banner of Popular Party's candidate Pablo Casado in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Tuesday, April 16, 2019 photo, Popular Party's candidate Pablo Casado, top and center, smiles while posing for snapshots with supporters after an election campaign event in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Tuesday, April 16, 2019 photo, Popular Party's candidate Pablo Casado, top and center, smiles while posing for snapshots with supporters after an election campaign event in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Saturday, April 6, 2019 photo, Spain's far-right Vox Party candidate Santiago Abascal smiles to supporters during a VOX event in Leganes, outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. . (AP PhotoBernat Armangue)

In this Saturday, April 6, 2019 photo, Spain's far-right Vox Party candidate Santiago Abascal smiles to supporters during a VOX event in Leganes, outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. . (AP PhotoBernat Armangue)

In this Tuesday April 23, 2019 photo, the torn remnants of election posters by UnidasPodemos candidate for parliament Alberto Garzon and the right wing Falange party are seen on a wall in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoPaul White)

In this Tuesday April 23, 2019 photo, the torn remnants of election posters by UnidasPodemos candidate for parliament Alberto Garzon and the right wing Falange party are seen on a wall in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoPaul White)

In this Saturday, April 6, 2019 photo, voters smoke a cigarette at the end of an event hosted by Spain's far-right Vox Party in Leganes, on the outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Saturday, April 6, 2019 photo, voters smoke a cigarette at the end of an event hosted by Spain's far-right Vox Party in Leganes, on the outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this April 10, 2019 photo, Vox banners hang from a bar in Brazatortas, on the edge of the Alcudia valley, central Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this April 10, 2019 photo, Vox banners hang from a bar in Brazatortas, on the edge of the Alcudia valley, central Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Monday, April 15, 2019 photo, Spanish retirees wait for the arrival of Prime Minister and Socialist Party candidate Pedro Sanchez at an election campaign event in Leganes, outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Monday, April 15, 2019 photo, Spanish retirees wait for the arrival of Prime Minister and Socialist Party candidate Pedro Sanchez at an election campaign event in Leganes, outskirts of Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Sunday, March 24, 2019 photo, activists paint an "estelada" or independence flag on a water tank near Barcelona, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoEmilio Morenatti)

In this Sunday, March 24, 2019 photo, activists paint an "estelada" or independence flag on a water tank near Barcelona, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoEmilio Morenatti)

In this Wednesday, April 24, 2019 photo, Podemos party leader Pablo Iglesias, delivers a speech during an electoral meeting in Barcelona, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years.  (AP PhotoEmilio Morenatti)

In this Wednesday, April 24, 2019 photo, Podemos party leader Pablo Iglesias, delivers a speech during an electoral meeting in Barcelona, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoEmilio Morenatti)

In this Wednesday, April 24, 2019 photo, a banner of Spanish election runner Citizens party's Albert Rivera is seen in the background in a subway station, in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)

In this Wednesday, April 24, 2019 photo, a banner of Spanish election runner Citizens party's Albert Rivera is seen in the background in a subway station, in Madrid, Spain. A substantial pool of undecided voters and a right-wing splintered by a rising populist party are making confident predictions harder to come by as Spain prepares for its third parliamentary election in less than four years. (AP PhotoBernat Armangue)