Mulvaney's bid to join impeachment lawsuit challenged

White House acting chief of staff Mick Mulvaney is facing opposition in his bid to join a lawsuit over testimony in the House impeachment inquiry.

Mulvaney on Friday asked to sign onto a lawsuit asking a court to decide whether he must comply with a congressional subpoena. The lawsuit was brought by Charles Kupperman, Trump's former deputy national security adviser. Like Kupperman, Mulvaney has defied a subpoena from impeachment investigators.

In a court filing Monday, Kupperman's attorney, Charles Cooper, said Mulvaney should not be allowed to join the lawsuit.

Cooper says there are important distinctions between Mulvaney and Kupperman's situations, including that Mulvaney has already spoken publicly about issues central to the impeachment inquiry. Cooper also noted that Mulvaney still works for Trump, while Kupperman does not.