The top U.S. diplomat in Ukraine arrived in the Capitol Tuesday morning to be interviewed as part of the Democrats' fast-advancing impeachment investigation, the latest in a string of witnesses to testify on 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 dealings with Kiev.



William Taylor, the U.S. chargé d'affaires of Ukraine, had expressed concerns last month that Trump had crossed a line by threatening to withhold U.S. military aid to Ukraine to pressure the country's president, Volodymyr Zelensky, for political favors.



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Taylor did not comment as he passed reporters on his way to the room.



A career diplomat who has worked under numerous presidents of both parties, Taylor last month had warned several State Department colleagues of a "nightmare scenario" if it came to light that Trump and his allies — notably his personal lawyer 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 — had dangled the military aid to pressure Zelensky to find dirt on the president's political opponents.



In a Sept. 8 text to 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, former special envoy to Ukraine, and Gordon Sondland, U.S. ambassador to the European Union, Taylor worried that Ukraine would make good on providing an unspecified "interview," but Trump would renege on the military aid.



"The nightmare is they give the interview and don't get the security assistance," Taylor texted to Volker and Sondland, according to the transcripts provided to Congress by Volker earlier in the month. "The Russians love it. (And I quit)."



The next day, Taylor warned Sondland that "it's crazy to withhold security assistance for help with a political campaign."



Sondland, a wealthy hotel magnate and Trump donor with no previous diplomatic experience, pushed back, saying Taylor was "incorrect" in surmising Trump's intentions.



"The president has been crystal clear no quid pro quo's of any kind," Sondland wrote.

An official working on the impeachment inquiry said the House Intelligence Committee subpoenaed Taylor this morning after the State Department allegedly sought to "direct" the diplomat "not to appear for his scheduled deposition," or seek to limit his testimony.

"As is required of him, Ambassador Taylor is now complying with the subpoena and answering questions from both Democratic and Republican Members and staff," the official said.

Both Volker and Sondland have already testified as part of the impeachment investigation, as has Marie Yovanovitch, the former U.S. ambassador to Ukraine who was removed in May after voicing concerns over Giuliani's pressure campaign.



Taylor is the most recent witness to testify in the Democrats' month-old impeachment inquiry, which was formally 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.) in response to allegations from a government whistleblower that Trump had dangled almost $400 million in military aid to Ukraine if Zelensky would investigate the son of 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.



The White House has said it will refuse to cooperate in the investigation, saying it's illegitimate. But Democrats have issued numerous subpoenas to break the stonewalling, and Taylor is just the latest witness to defy the administration's attempted blockade.



Trump has minced no words in his criticism of the ongoing inquiry, accusing Democrats of conducting a "witch hunt" designed with the sole purpose of sinking his reelection. Tuesday morning, just before Taylor arrived, Trump called the process "a lynching."



"So some day, if a Democrat becomes President and the Republicans win the House, even by a tiny margin, they can impeach the President, without due process or fairness or any legal rights," he tweeted. "All Republicans must remember what they are witnessing here - a lynching. But we will WIN!"



Democrats have long condemned Trump's frequent race-based allusions, and wasted no time rebuking the president for his latest comments.



"Whenever his back is against the wall, a racial bomb is what we know him to [throw]," Rep. Karen Bass Karen Ruth BassOvernight 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 Democrats push to limit transfer of military-grade gear to police Outrage erupts over Breonna Taylor grand jury ruling MORE (D-Calif.), head of the Congressional Black Caucus, said Tuesday morning.

Olivia Beavers contributed.