The White House defense against Democrats’ impeachment inquiry suffered a major blow Thursday with acting 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’s admission that aid for Ukraine was linked to 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 desire for the country to pursue a political probe related to the 2016 election.

The stunning admission marked the first time a White House official had publicly undermined Trump's repeated denials of any quid pro quo. It also coincided with a host of current and former administration officials raising concerns during closed-door testimony about the administration’s Ukraine policy.

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Mulvaney indicated he felt the behavior was nothing out of the ordinary, telling reporters to “get over it.”

“There’s going to be political influence in foreign policy,” Mulvaney said during a rare press conference in the White House briefing room. “That’s going to happen.”

His comments threaten to erode the administration's narrative that there was no quid pro quo between Trump and Ukraine and may bolster the Democrats' impeachment probe.

Mulvaney walked back his remarks hours later as unease built over his press briefing, where he indicated that the administration had held up almost $400 million in military assistance to Ukraine in part because Trump wanted Kiev to investigate an unsubstantiated allegation about Ukraine’s involvement in the hack of the Democratic National Committee (DNC) server in 2016. The theory undercuts Russian involvement in the hack and the U.S. intelligence community assessment about election interference by Moscow.

Mulvaney blamed the media for misconstruing his comments and advancing “a biased and political witch hunt against President Trump.”

“The president never told me to withhold any money until the Ukrainians did anything related to the server,” Mulvaney said in a statement. “The only reasons we were holding the money was because of concern about lack of support from other nations and concerns over corruption."

Trump later defended Mulvaney, telling reporters in Texas that while he did not watch the press briefing, he has “a lot of confidence” in his acting chief of staff.

Trump has repeatedly claimed there was “no quid pro quo” in his dealings with Ukraine. Under fire over revelations he pressed Ukraine’s leader to investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE, Trump has insisted his reason for putting a hold on security assistance to Ukraine was out of concern European countries were not contributing enough.

Mulvaney argued Thursday that efforts by Trump to urge Ukraine to investigate the Bidens had nothing to do with the aid being withheld.

But he did acknowledge that the aid was tied in part to Trump’s desire for help investigating the hack of the DNC server — something viewed by the president’s critics as a politically motivated effort to undercut the U.S. intelligence community's conclusion that Russia interfered with the election in 2016 to benefit his campaign.

“The look back to what happened in 2016 certainly was part of the things that he was worried about in corruption with that nation. And that is absolutely appropriate,” Mulvaney told reporters at the White House Thursday, arguing it was related to an ongoing Department of Justice (DOJ) investigation.

When asked if what he described was a quid pro quo, Mulvaney responded, "We do that all the time with foreign policy,” comparing it to when the administration cut off aid to Central American countries to pressure them to curb migrant flows.

A senior DOJ official, when asked about Mulvaney’s remarks, said, “If the White House was withholding aid in regards to the cooperation of any investigation at the Department of Justice, that is news to us.”

Trump allies scrambled Thursday afternoon to distance themselves from Mulvaney’s comments.

“The President's legal counsel was not involved in acting chief of staff Mick Mulvaney's press briefing,” Trump’s personal attorney Jay Sekulow Jay Alan SekulowNow, we need the election monitors Judge denies Trump's request for a stay on subpoena for tax records Judge throws out Trump effort to block subpoena for tax returns MORE said in a statement Thursday afternoon.

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One House GOP aide mused that Mulvaney’s performance was “so bad that it’s plausible he was trying to get fired.”

Fox News host and Trump confidant Sean Hannity Sean Patrick HannityFormer Florida attorney general calls Kyle Rittenhouse 'a little boy out there trying to protect his community' Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Cruz: Trump should nominate a Supreme Court justice next week MORE called Mulvaney’s interpretation of events “idiotic.”

“I just think he’s dumb, I really do,” Hannity said. “I don’t even think he knows what he’s talking about.”

Elie Honig, a legal analyst and former federal prosecutor, said Mulvaney may have been trying to grapple with the drips of news coming out of the impeachment inquiry suggesting there was some kind of quid pro quo.

“I think they’re calculating what they can and cannot contest. And I think they’re realizing that it’s getting harder and harder to contest the fact that there was some kind of quid pro quo,” Honig said. “I think they’re trying to frame the quid pro quo in a way that will be the least damaging to Trump.”

Mulvaney’s comments reverberated across Capitol Hill, where lawmakers leading the impeachment inquiry into Trump were meeting during the day with Gordon Sondland, the U.S. ambassador to the European Union.

“I think Mr. Mulvaney’s acknowledgment means that things have gone from very, very bad to much, much worse,” House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) told reporters.

Rep. Eric Swalwell Eric Michael SwalwellHouse to vote on resolution affirming peaceful transition of power Swalwell calls for creation of presidential crimes commission to investigate Trump when he leaves office 'This already exists': Democrats seize on potential Trump executive order on preexisting conditions MORE (D-Calif.) tweeted that Mulvaney had “co-signed [Trump’s] confession to the crime.”

By Thursday evening, Mulvaney was doing damage control.

“There was never any connection between the funds and the Ukrainians doing anything with the server -- this was made explicitly obvious by the fact that the aid money was delivered without any action on the part of the Ukrainians regarding the server,” he said in the statement walking back his previous remarks.

Mulvaney confirmed Thursday that he was not on the July 25 call between Trump and Ukrainian President Volodymyr Zelensky, but he has faced questions about the White House's handling of the controversy in recent weeks, including how a transcript of the call ended up on a secure server meant for matters of national security.

A rough transcript of the call released by the White House last month showed Trump asking his Ukrainian counterpart to “look into” the Bidens and if he could “do us a favor” by probing a company with ties to the DNC server hack.

Emerging details about the call sparked a formal impeachment inquiry launched late last month by House Democrats, even as Trump and his aides have publicly argued there was nothing improper about the conversation.

Text messages provided to House investigators by the former State Department special envoy to Ukraine showed Trump administration officials indicating that a White House meeting between Trump and Ukraine’s leader would be predicated on Kiev launching investigations related to 2016 election interference and Burisma, a Ukrainian energy company where Biden's son Hunter served on the board.

House lawmakers have recently heard testimony from current and former administration officials — former U.S. Ambassador to Ukraine Marie Yovanovitch; Fiona Hill, a former Russia expert on the National Security Council; Michael McKinley, a former top aide to Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE; former State Department official George Kent; and Sondland — who have said they were concerned about efforts by Trump’s personal attorney 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 and others in the administration to circumvent traditional channels and pursue their agenda in Ukraine.

Trump and the White House have sought to downplay their testimony, chalking it up to political bias.

“What you’re seeing now, I believe, is a group of mostly career bureaucrats who are saying, ‘You know what? I don’t like President Trump’s politics, so I’m going to participate in this witch hunt that they’re undertaking on the Hill,’ ” Mulvaney said Thursday.

Sondland told House investigators on Thursday that Trump told him there was “no quid pro quo” related to the security assistance in Ukraine and that he wanted “nothing” from Ukraine, according to a copy of his opening remarks.

Sondland also insisted he never participated in an effort to coerce a foreign government into conducting investigations for the purpose of influencing an upcoming election, and that doing so “would be wrong.”

The president has hit Democrats over the lack of transparency in their hearings, and claimed he does not know most of the current and former administration officials meeting with lawmakers.

But the parade of witnesses has underscored the extent to which the White House’s strategy to stonewall the impeachment inquiry has fallen short.

In addition to those who have already met with lawmakers, five more administration officials are scheduled to give closed-door testimony next week. Mulvaney also faces a Friday deadline to comply with a subpoena from House Democrats for documents related to their investigation.

Scott Wong contributed.