WASHINGTON — White House acting chief of staff Mick Mulvaney is facing opposition, including from a former Trump administration adviser, in his bid to join a lawsuit over testimony in the House impeachment inquiry.

The opposition from Charles Kupperman, the former deputy national security adviser who sued over a subpoena from the House, underscores the lack of a unified approach from Trump administration officials in responding to the impeachment inquiry, which enters a critical public phase this week. Several people have testified in defiance of White House orders, while others have skipped their scheduled appearances.

Mulvaney on Friday asked to sign onto Kupperman’s lawsuit, which asked a court to decide on compliance with congressional subpoenas. Kupperman said he faced two conflicting directives — one from Congress telling him he must testify, the other from the White House telling him he is immune from having to testify.

Like Kupperman, Mulvaney has defied a subpoena from impeachment investigators. Lawyers for Mulvaney said their arguments raised similar issues that belong in the same lawsuit.

But in a court filing Monday, Charles Cooper, who represents both Kupperman and former national security adviser John Bolton, said there were important distinctions between Mulvaney and Kupperman’s situations, including that Mulvaney has already spoken publicly about issues central to the impeachment inquiry from the podium of the White House briefing room.

“Plaintiff, in contrast, has never publicly disclosed information relating to any of his official duties, including the matters under investigation by the House,” Cooper wrote of Kupperman.

Cooper said there is a “serious question” as to whether Mulvaney waived the immunity protection with his public comments. He also noted that Mulvaney still works for Trump, while Kupperman does not.

House Democrats also opposed Mulvaney’s effort to join the suit.

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