Via Trump Train News:

Chief of Staff Mick Mulvaney has filed a separate suit to fight the subpoena from House Democrats compelling him to testify in the impeachment inquiry.

According to The Hill:

Mick Mulvaney, President Trump's acting chief of staff, told a federal judge that he is withdrawing from his effort to join a former White House aide's lawsuit against House Democrats and intends to file his own case in an effort to fight a subpoena as part of the impeachment inquiry.

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.

Mulvaney is arguing that both Republican and Democrat administrations have viewed close advisors as being immune to Congressional subpoenas.