Two diplomats at the center of the Ukraine controversy are now set to testify before House lawmakers as part of Democrats’ impeachment inquiry, despite significant pushback from both the State Department and the White House.

Marie Yovanovitch, the former U.S. envoy to Kiev and someone President Trump has privately called "bad news," arrived on Capitol Hill Friday for a potentially explosive transcribed interview with lawmakers and staff.

TRUMP ALLED FORMER UKRAINE DIPLOMAT 'BAD NEWS'...AND NOW SHE'S SET TO TESTIFY

Trump and his allies have sought to paint Yovanovitch as a rogue employee with an anti-Trump bias. She was ousted in May amid alleged attempts by Trump attorney Rudy Giuliani to press Ukraine into investigating Democratic presidential candidate Joe Biden and his son. Those efforts ultimately led to the impeachment inquiry after the revelation of a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky -- in which Trump asked the premier to “look into” the allegations about Biden's conduct in the country.

While Yovanovitch, a long-time diplomat, has been praised by her colleagues as a "top-notch diplomat," Trump depicted her in the call with Zelensky as “bad news” and someone who is “going to go through some things.” Giuliani and other critics have accused her of working to undermine Trump’s interests.

Former U.S. Ambassador to Ukraine Marie Yovanovitch, center, is set to testify on Friday on Capitol Hill. (Mikhail Palinchak, Presidential Press Service Pool Photo via AP)

Also Friday, U.S. Ambassador to the European Union Gordon Sondland announced that he will testify before Congress. The announcement comes a week after the State Department directed him not to appear before lawmakers at a scheduled deposition. The shift comes after House Democrats on Wednesday subpoenaed him to appear before the joint committees to testify. He said in a statement that he is scheduled to appear on Thursday.

TRUMP ADMINISTRATION BLOCKS AMBASSADOR FROM TESTIFYING IN HOUSE IMPEACHMENT DEPOSITION

“Notwithstanding the State Department’s current direction to not testify, Ambassador Sondland will honor the Committees’ subpoena, and he looks forward to testifying on Thursday,” the statement said. "Ambassador Sondland has at all times acted with integrity and in the interests of the United States. He has no agenda apart from answering the Committees’ questions fully and truthfully.”

Trump had dismissed the deposition as a “kangaroo court,” while Democrats warned that a failure of Sondland and other witnesses to appear was evidence of obstruction of Congress.

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“I would love to send Ambassador Sondland, a really good man and great American, to testify," Trump tweeted, "but unfortunately he would be testifying before a totally compromised kangaroo court, where Republican’s rights have been taken away, and true facts are not allowed out for the public to see."

Democrats claim that Trump used $400 million in military aid as leverage in a quid pro quo for the Ukrainians to investigate a political opponent, former Vice President Joe Biden, and opened an impeachment inquiry, firing off a raft of subpoenas within a few days to top administration and White House officials. While a call transcript shows the president urging a Biden-related investigation, Trump denies any 'quid pro quo' claims.

Sondland is a wealthy hotelier, philanthropist and contributor to political campaigns. He is a key figure as he had texted with Bill Taylor, a top U.S. diplomat in Ukraine, arguing over allegations of a quid pro quo.

In early September, Taylor wrote: "Are we now saying that security assistance and WH meeting are conditioned on investigations?"

Days later, he followed up: "As I said over the phone, I think it's crazy to withhold security assistance for help with a political campaign."

Sondland responded, defending the president: "Bill, I believe you are incorrect about President Trump's intentions. The President has been crystal clear: no quid pro quo’s of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign.”

It is not clear how much of the change in Sondland’s position is his own, and how much the Trump administration has shifted on the question of whether he should appear. Axios reported Friday that Republicans close to Trump encouraged the president to let the ambassador come before the committees and that his allies believe Sondland's testimony will be helpful to them.

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But the White House has been aggressive in its pushback against the impeachment inquiry, which it sees as illegitimate and politically motivated. On Wednesday the White House issued a defiant letter saying it would not cooperate with the inquiry.

"President Trump and his administration reject your baseless, unconstitutional efforts to overturn the democratic process," the letter stated. "Your unprecedented actions have left the president with no choice. In order to fulfill his duties to the American people, the Constitution, the Executive Branch, and all future occupants of the Office of the Presidency, President Trump and his administration cannot participate in your partisan and unconstitutional inquiry under these circumstances."

Fox News' Gregg Re, Lillian LeCroy, Nick Kalman, Chad Pergram and The Associated Press contributed to this report.