Acting White House chief of staff Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE on Friday defied a subpoena for his testimony in the House impeachment probe at the last minute, in what is likely the Democrats’ final effort to hear privately from witnesses about President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE’s contacts with Ukraine before their inquiry goes public.

Mulvaney did not appear for his closed-door deposition in the Capitol, citing White House claims of immunity, according to an official working on the inquiry. The official said Mulvaney's outside counsel informed House investigators “one minute” before his scheduled deposition that he would not appear.

“This morning, one minute before his scheduled deposition was to start, Mr. Mulvaney’s outside counsel informed us that his client had been directed by the White House not to comply with the duly authorized subpoena and asserted 'absolute immunity,' " the official said in a statement.

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Democrats said that the refusal to testify will add to their case arguing that the Trump administration sought to obstruct their impeachment inquiry.

“If the White House had evidence to contest those facts, they would allow Mr. Mulvaney to be deposed, as three chiefs of staffs of President Clinton were during his impeachment proceedings,” the official added.

Trump, when asked by reporters Friday why Mulvaney would not appear, said he did not want to “give credibility to a corrupt witch hunt.”

“I’d love to have Mick go up, frankly,” he added.

Mulvaney was among the dwindling group of witnesses Democrats have sought to bring in to discuss whether the president used nearly $400 million in financial aid as leverage to press Kiev into opening two politically motivated investigations that would benefit Trump. And like a number of other top, Trump-appointed administration officials, he sided with the White House claims of executive privilege over House Democrats’ subpoena.

Mulvaney has served as the director of the White House Office of Management and Budget (OMB), which is charged with executing the federal spending laid out by Congress. In that capacity, the onetime conservative House member has been seen as a key witness who can shed light on the extent of Trump’s direct involvement in the aid holdup.

“Mr. Mulvaney admitted from the White House briefing room that the president withheld vital military aid in order to pressure Ukraine to conduct investigations that would benefit the President’s personal and political interests, not the national interest,” the official working on the impeachment inquiry said, citing multiple witness testimonies.

On Saturday, after news broke that Mulvaney was seeking to join a lawsuit challenging the subpoena, an official working on the inquiry released a new statement.

"In personally directing his Acting Chief of Staff, Mick Mulvaney, to defy a lawful subpoena and refuse to testify before the Committees, President Trump has added to the growing body of evidence documenting his attempts to obstruct the House's impeachment inquiry," the official said.

"Mr. Mulvaney has already publicly admitted from the White House briefing room that the President withheld vital military aid in order to pressure Ukraine to conduct investigations intended to benefit the President’s personal and political interests. For Mr. Mulvaney to now claim that he cannot discuss under oath without court approval the same matters which he has already discussed publicly doesn’t fool anyone — but it does lay bare the superficial, inconsistent and insupportable nature of the President’s obstruction of Congress."

William Taylor, the top diplomat to Ukraine, told House investigators last month that an OMB staffer notified him that the aid was being withheld, and that the request was made by the president.

Taylor testified that during a July 18 phone call, in what had considered “an otherwise normal meeting,” the OMB staffer told the participants on the phone call that “her boss had instructed her not to approve any additional spending of security assistance for Ukraine until further notice.”

“All that the OMB staff person said was that the directive had come from the president to the chief of staff to OMB. In an instant, I realized that one of the key pillars of our strong support for Ukraine was threatened,” Taylor testified, according to his deposition transcript released this week.

Fiona Hill, Trump’s former leading Russia expert, also implicated Mulvaney in the administration’s pressure campaign in Ukraine. Hill reportedly told investigators that John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE, Trump’s former national security adviser, was so alarmed by what he heard about Trump’s contacts with Kiev that he instructed her to notify the chief lawyer for the National Security Council about the efforts of Trump's personal lawyer Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, Mulvaney and Gordon Sondland, the U.S. ambassador to the European Union.

“I am not part of whatever drug deal Sondland and Mulvaney are cooking up,” Bolton told Hill, according to her reported testimony.

Sondland this week drew widespread attention when he revised his initial testimony to include an acknowledgement that he himself had told a top Ukrainian official that the release of the military aid was “likely” dependent on Kiev’s willingness to open the investigations Trump had sought.

Trump and his GOP allies in Congress have argued there was no quid pro quo in the administration’s dealing with Ukraine, since the aid was ultimately delivered without Kiev opening those investigations.

Mulvaney had complicated that narrative last month, however, when he told reporters that the reluctance of Ukrainian leaders to investigate the 2016 election was, indeed, the reason the aid was not initially delivered.

“[Did] he also mention to me, in the past, that the corruption related to the DNC server?” Mulvaney said on Oct. 17. “Absolutely, no question about that. But that’s it. And that’s why we held up the money.”

Mark Sandy, a top official in the OMB, also did not appear despite being slated to testify.