A day after saying he would take the debate to court, acting White House chief of staff Mick Mulvaney says he will no longer pursue a lawsuit over whether he should comply with a congressional subpoena to testify in the impeachment inquiry against his boss, President Trump.

He will instead "rely on the direction of the president" and the Justice Department. In other words, Mulvaney — a key witness — won't comply with a congressional subpoena.

Since the impeachment inquiry began in late September, the White House has ordered Trump officials not to testify before Congress. In a letter to the White House last week, the Justice Department's Office of Legal Counsel backed up the president's assertion that his advisers have "absolute immunity" from congressional subpoenas in the impeachment probe. Assistant Attorney General Steven Engel argued that since Mr. Trump's closest advisers "serve as his alter egos," compelling their testimony would "undercut the 'independence and autonomy' of the Presidency."

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Since that letter was sent, the number of witnesses who have refused to testify has more than tripled.

Mulvaney would be a key witness. The impeachment inquiry is centered around whether Mr. Trump committed a politically charged quid pro quo and temporarily withheld U.S. aid from Ukraine as a means of pressuring the foreign country to launch investigations that could benefit the president's reelection campaign. It is illegal to seek or accept foreign assistance in U.S. elections.

In addition to his interim role as chief of staff, Mulvaney is also the head of the Office of Management and Budget, which was involved in delaying the aid. And during a press conference last month, Mulvaney appeared to admit — and later denied — that Mr. Trump had offered a quid pro quo to the Ukrainian president: the release of military aid in exchange for an investigation into an unsubstantiated claim that a server related to the hacking of the Democratic National Committee is in Ukraine.

Acting White House Chief of Staff Mick Mulvaney in the Oval Office at the White House July 9, 2019. Getty Images

Before reversing course, Mulvaney had asked to join a former Trump adviser's lawsuit on Saturday that would have let the courts decide whether he should testify. Then on Monday, after the originator of the lawsuit, Charles Kupperman asked he withdraw that request, Mulvaney said he would instead file his own case that would be related to Kupperman's. On Tuesday, he abandoned any legal effort that could result in his appearing before Congress.

The impeachment committees will hold their first televised hearings this week, featuring testimony on Wednesday from William Taylor, the top U.S. diplomat in Ukraine, and George Kent, the deputy assistant secretary of state for European and Eurasian Affairs.

Stefan Becket, Clare Hymes and Jon Legare contributed reporting.