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Mulvaney last week had asked to join the original case, which would have put him in the awkward position of essentially suing his own boss. That suit was filed by Charles Kupperman, a former deputy national security adviser who was essentially asking the court for guidance on whether the president could order current and former White House officials to defy congressional subpoenas.

But House Democrats decided they didn't want to resolve that question in the courts for Kupperman's testimony and ultimately withdrew his subpoena.

In a filing on Monday, they asked the court to block Mulvaney from joining Kupperman's lawsuit, arguing that it was moot and that the Trump aide was facing different circumstances.

Kupperman also opposed Mulvaney's effort, arguing that the two had different aims in taking the subpoenas to the courts.

Mulvaney argued in his proposed lawsuit that the Justice Department's legal advisers under both Democratic and Republican administrations have advised that the president and his close advisers are immune to congressional subpoena.

"In short, there apparently is no controlling judicial authority that definitively undermines the Executive Branch’s consistent bipartisan conclusion that close personal advisors of the President are bound to abide by the President’s assertions of immunity, even in the face of congressional subpoenas for testimony," Mulvaney's lawyers said in their original filing.