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 was deeply involved in the White House’s dealings with Ukraine, coordinating more with diplomats and other administration officials than previously known, according to a report in The Washington Post.

The Post reports that Mulvaney worked closely with diplomats Gordon Sondland and Kurt Volker Kurt VolkerGOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports Live coverage: Senators enter second day of questions in impeachment trial MORE and Energy Secretary Rick Perry Rick PerryEnergy secretary questions consensus that humans cause climate change OVERNIGHT ENERGY: Democrats push resolution to battle climate change, sluggish economy and racial injustice | Senators reach compromise on greenhouse gas amendment stalling energy bill | Trump courts Florida voters with offshore drilling moratorium OVERNIGHT ENERGY: Trump signs major conservation bill into law | Senate votes to confirm Energy's No. 2 official | Trump Jr. expresses opposition to Pebble Mine project MORE at a time when the White House was pressuring Kiev 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, one of the leading contenders to face 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 in the 2020 election.

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The White House did not respond to a request for comment from The Hill, but some of Mulvaney’s defenders have said he knew very little of the White House’s efforts regarding Ukraine and was mostly organizing meetings for Trump.

“I don’t remember any substantive conversation with Mick. I don’t remember him approving, disapproving, getting involved, having an interest,” 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, Trump’s personal lawyer who has been deeply entangled in the scandal, told the Post. “Mulvaney was not a big player in this. I dealt with Volker and Sondland.”

However, current and former officials told the Post that Mulvaney played a large role in the unfolding crisis. The Post's reporting confirms one Democratic lawmaker's report coming out of closed-door testimony on Tuesday.

Officials told the Post that Mulvaney often met with Sondland and that details of their conversations were kept from then-national security adviser 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 and others who had expressed concerns about the White House’s efforts regarding Ukraine.

Mulvaney also allowed meetings between Trump and Giuliani as Giuliani publicly discussed his efforts to lobby Kiev into investigating conspiracy theories about the 2016 election and dig up dirt on Biden.

According to the report, Mulvaney also, at Trump’s behest, put a hold on nearly $400 million in military aid to Ukraine in the weeks before Trump held a July 25 phone call with Ukrainian President Volodymyr Zelensky in which he pressured him to work with Giuliani and Attorney General William Barr Bill BarrFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Why a backdoor to encrypted data is detrimental to cybersecurity and data integrity FBI official who worked with Mueller raised doubts about Russia investigation MORE to investigate Biden, a phone call that has, in part, triggered the House’s impeachment investigation.

Mulvaney reportedly told staff that the money to Kiev could be turned on and off without legal repercussions, though some questioned the legality of putting the aid on hold.

Officials told the Post that the shadow campaign to get Kiev to politically benefit Trump could not have been conducted without Mulvaney’s facilitation of meetings and blocking the aid.

A former South Carolina congressman who was installed as chief of staff in January, Mulvaney has appeared more eager than his predecessors to follow Trump’s commands and has shirked past efforts to curtail the number of staffers who have direct access to the president.

“Mulvaney has really abdicated the most important duty of any White House chief of staff, and that’s to tell the president hard truths,” Chris Whipple, author of “The Gatekeepers,” a book about past White House chiefs of staff, told the Post. “He really should have thrown his body in front of that phone call with Zelensky.”

A declassified copy of a whistleblower complaint regarding the call said that a future phone call or meeting between the two presidents “would depend on whether Zelensky showed willingness to ‘play ball’ ” and that “multiple White House officials with direct knowledge” of the call were alarmed that Trump appeared to be using his office for his personal political gain and sought to bury the transcript of the conversation.

Democrats have pointed to the decision to block the aid as evidence that Trump may have abused his power to leverage the aid in an attempt to pressure Zelensky to comply with his request to investigate the Bidens.

Zelensky said last week there was “no blackmail” during a phone call and that he and Trump did not discuss the military aid.