WASHINGTON — White House Chief of Staff Mick Mulvaney signaled Monday night that he will file his own lawsuit challenging a congressional subpoena for testimony in the impeachment inquiry.

Mulvaney earlier filed a motion to join an existing lawsuit filed by a former Trump administration official, Charles Kupperman, over a similar subpoena issue. But lawyers for Kupperman and the House Intelligence Committee urged the judge not to let that happen.

Mulvaney sought to intervene in a suit filed late last month by former White House national security adviser Charles Kupperman, which named both the House and the President Trump as defendants. Faced with a subpoena to testify before the House and also a letter from the White House counsel instructing him not to do so, Kupperman asked a federal court to rule which command he should obey.

Last week, the House withdrew the subpoena and urged the judge to dismiss the lawsuit as moot. But before Federal District Court Judge Richard Leon had ruled on that, Mulvaney asked to intervene in the suit, arguing that he, too, faced the same dilemma. But seeking to join the case, Mulvaney put himself in the odd position of suing his own boss, even though he claimed to be challenging only the House subpoena.

Both Kupperman and the House told the judge that Mulvaney should file his own case. Kupperman's lawyers said he didn't care which way the judge ruled on whether he should have to comply with a congressional subpoena or should obey the White House order to disregard it. By contrast, they said, Mulvaney has made it clear he wants the judge to rule against the House.

By publicly saying there was a quid pro quo in the phone call, Mulvaney might have already waived, or given up, any claim he has to immunity from the subpoena, Kupperman's lawyers said. They added that allowing Mulvaney into the case could bog it down in new issues that would drag the case out, when it was likely to be dismissed because the House withdrew Kupperman's subpoena.

After a brief telephone conference call Monday evening, Mulvaney's lawyers filed a notice withdrawing their motion to join the Kupperman's case and saying they would file their own lawsuit.