A group of White House budget officials allied with Acting Chief of Staff Mick Mulvaney are reportedly teaming up to form a "firewall" against House Democrats' impeachment inquiry into President Donald Trump, an effort legal experts and lawmakers condemned as obstruction of Congress and further evidence that the administration is attempting to conceal evidence of wrongdoing.

The Washington Post reported late Sunday that "Russell Vought, a Mulvaney protege who leads the White House Office of Management and Budget, intends a concerted defiance of congressional subpoenas in coming days, and two of his subordinates will follow suit."

Vought, who has called the impeachment probe a "sham," said he will defy a subpoena to testify before House investigators on Wednesday. In addition to Vought, OMB officials Brian McCormack and Michael Duffey—scheduled to testify Monday and Tuesday, respectively—have also said they will not comply with the House committees' request to testify.

The White House budget office is central to the House impeachment investigation because of the agency's reported role in withholding aid to Ukraine on the president's orders. Around a week before Trump's July 25 call with Ukrainian leader Volodymyr Zelensky, the U.S. president instructed OMB—then headed by Mulvaney—to withhold $400 million in aid to Ukraine.

During a press briefing last month, Mulvaney admitted that the budget office withheld the aid as part of an effort to pressure Ukraine to investigate Democrats.

"We do that all the time with foreign policy," Mulvaney told reporters. "Get over it."

Michael Bromwich, former Justice Department inspector general, said that by refusing to comply with House committees, the White House budget officials are acting as "co-conspirators in the WH's obstruction."

"They should all be held in contempt once they defy subpoenas," Bromwich tweeted.

No more playing around with these co-conspirators in the WH's obstruction. They should all be held in contempt once they defy subpoenas. https://t.co/93wEbXm0ma — Michael R. Bromwich (@mrbromwich) November 4, 2019

According to the Post, the officials' "anticipated defiance toward impeachment investigators comes as Trump has grown enraged that so many of his 'employees,' as he refers to them, are going to Capitol Hill and testifying."

"The president has asked for copies of witness statements so he can decide how to criticize them, complained that his lawyers are not doing enough to stop people from talking, and even encouraged members of Congress to question the credibility of people working in his own administration," the Post reported.

None of the four Trump administration officials scheduled to testify Monday—OMB's McCormack, Mulvaney adviser Robert Blair, and National Security Council lawyers John Eisenberg and Michael Ellis—are expected to show up, CNN reported late Sunday.

"What are these White House officials hiding from the American people?" asked Rep. Ted Lieu (D-Calif.), a member of the House Judiciary Committee. "Also, this is yet more evidence of obstruction of Congress, which is an impeachable offense."