Democrats say acting White House chief of staff Mick Mulvaney Mick MulvaneyMick Mulvaney to start hedge fund Fauci says positive White House task force reports don't always match what he hears on the ground Bottom line MORE will be the key to explaining just how closely involved President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE was to the effort led by his personal lawyer, Rudy Giuliani Rudy GiulianiGrand jury adds additional counts against Giuliani associates Lev Parnas and and Igor Fruman Juan Williams: Breaking down the debates Giuliani criticizes NYC leadership: 'They're killing this city' MORE, to pressure Ukraine to investigate former Vice President Joe Biden Joe BidenMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE.

One of the biggest weaknesses of the House managers' case is that they cannot point to firsthand testimony that Trump directed his subordinates to withhold military aid to use as a bargaining chip with Ukraine in exchange for a probe of Biden and his son. But Democrats believe Mulvaney was privy to the most essential conversations.

“I think Mulvaney knows way more than most people estimate,” said Senate Minority Whip Dick Durbin Richard (Dick) Joseph DurbinMcConnell focuses on confirming judicial nominees with COVID-19 talks stalled Senate Republicans signal openness to working with Biden Top GOP senator calls for Biden to release list of possible Supreme Court picks MORE (D-Ill.).

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Durbin added that he sees Mulvaney as a more important witness than former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE, who has said he is willing to testify at the Senate trial.

Democrats say the centrality of Mulvaney to the case against Trump was made plain Saturday by Trump’s own lawyers, who attacked the House managers for not having a witness to directly link Trump to the pressure campaign against Ukraine.

Democratic senators point to Mulvaney’s Oct. 17 press conference where he admitted that the Trump administration had political reasons for withholding aid to Ukraine.

“I have news for everybody. Get over it. There’s going to be political influence in foreign policy,” Mulvaney declared in a statement that the House managers have played repeatedly on video during the trial.

White House deputy counsel Mike Purpura homed in on the testimony of U.S. Ambassador to the European Union Gordon Sondland Gordon SondlandGOP chairman vows to protect whistleblowers following Vindman retirement over 'bullying' Top Democrat slams Trump's new EU envoy: Not 'a political donor's part-time job' Trump names new EU envoy, filling post left vacant by impeachment witness Sondland MORE’s admission that he presumed Trump was behind the pressure campaign.

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Purpura argued Saturday that all Democrats have to support the “alleged link between the security assistance and investigations” were “Sondland’s assumptions and presumptions.”

After Trump’s lawyers rested their arguments for the day, Senate Minority Leader Charles Schumer Chuck SchumerVideo of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Graham signals support for confirming a Supreme Court nominee this year Pelosi orders Capitol flags at half-staff to honor Ginsburg MORE (D-N.Y.) said Mulvaney could certainly fill in missing details that even other requested witnesses, such as Bolton, may or may not know.

“There are people who have eyewitness accounts, the very four witnesses and the very four sets of documents that we have asked for,” Schumer said. “They made the argument that no one really knows what the president intended, but there are people who do know. Mick Mulvaney knows.”

House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Overnight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies MORE (D-Calif.), the lead House impeachment manager, pounced on the conspicuous omission of any mention of Mulvaney in Trump’s lawyers’ opening argument.

“One last thing that really stood out to me and that was something that wasn’t said. It was a name in fact that was never mentioned and that is Mick Mulvaney. There is no mention of the chief of staff,” Schiff said.

“What about Mulvaney?” he added. “When they say no witnesses could directly put words in the president’s mouth — well, first of all Gordon Sondland did — but what about Mick Mulvaney, who admitted in a press conference just like this, ‘Of course we did. It happens all the time. Get over it.’ "

“Why did they make no mention of Mick Mulvaney? Why would they have you look away from the fact that the president’s own chief of staff has admitted to the most pernicious part of this scheme, which is the withholding of military aid to get Ukraine to do these investigations?" he concluded.