The U.S. ambassador to the European Union arrived on Capitol Hill on Thursday morning to testify on the administration’s controversial dealings with Ukraine, becoming the latest witness in the Democrats’ quickly evolving impeachment investigation into 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.

Gordon Sondland, a wealthy hotel magnate, GOP mega-donor and late Trump convert, was escorted shortly after 9 a.m. into the secure room in the Capitol basement where three House committees — Intelligence, Oversight and Reform, and Foreign Affairs — are leading the impeachment inquiry launched by Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE (D-Calif.) just three weeks ago.

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Sondland was a central player in the administration’s campaign to pressure Ukrainian leaders to find dirt on Trump’s domestic political rivals. The campaign led to the ouster of Marie Yovanovitch, former U.S. ambassador to Ukraine, in May, apparently because she’d expressed concerns that the administration was seeking foreign help to influence elections at home.

Sondland did not comment as he was shuttled into the Capitol.

Democrats are eager to press Sondland on his involvement in the Ukraine affair, as they seek to determine whether the Trump administration used financial aid as leverage to press 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, a leading 2020 presidential contender, and his son, Hunter.

Sondland is the sixth witness to be deposed by the three committees since Pelosi announced the impeachment inquiry late last month. While Democrats, through much of the year, had focused their investigative efforts on Russian meddling in the 2016 elections, they’ve shifted their sights since a government whistleblower alleged that Trump threatened to withhold hundreds of millions of dollars in U.S. military aid to Ukraine if its president, Volodymyr Zelensky, declined to investigate the Bidens.

Text messages turned over to Congress this month by 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, the administration’s former special envoy to Ukraine, revealed a conversation last month between Sondland, Volker and William Taylor, currently the top U.S. diplomat in Ukraine, in which Taylor seemed to express grave concerns that Trump was hinging the release of U.S. military aid to Ukraine on Zelensky’s willingness to investigate the Bidens.

“As I said on the phone, I think it’s crazy to withhold assistance for help with a political campaign,” Taylor wrote in a Sept. 9 message.

Hours later — and reportedly after Sondland spoke directly with Trump — he pushed back on the suggestion, replying that Taylor was “incorrect.” The administration was interested in fighting Ukrainian corruption in general, Sondland said, not the Bidens in particular.

“The president has been crystal clear no quid pro quo’s of any kind,” Sondland wrote. “The president is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign. I suggest we stop the back and forth by text.”

The Wall Street Journal, however, reported this month that Sondland had told Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' The Hill's 12:30 Report: Ginsburg lies in repose CHC leaders urge Senate to oppose Chad Wolf nomination MORE (R-Wis.) that the administration had dangled the military aid in return for the investigation — a report that’s sure to be of interest to Democrats questioning Sondland on Thursday.

In his opening remarks, Sondland is expected to separate himself from the efforts of 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 say he took steps to connect Trump with Zelensky at the direction of the president. But he will deny knowledge of efforts to withhold aid as leverage and the extent to which the pressure campaign for a Biden probe went.

“Let me state clearly: Inviting a foreign government to undertake investigations for the purpose of influencing an upcoming U.S. election would be wrong. Withholding foreign aid in order to pressure a foreign government to take such steps would be wrong,” he will say, according to a copy of his opening remarks, which were obtained by The Hill.

“I did not and would not ever participate in such undertakings,” he will add. “In my opinion, security aid to Ukraine was in our vital national interest and should not have been delayed for any reason.”

--This report was updated at 10:09 a.m.