WASHINGTON — President Donald Trump delivered payback Friday to two key witnesses who testified in the House impeachment inquiry, firing Gordon Sondland, the U.S. ambassador to the European Union, and removing Lt. Col. Alexander Vindman from his White House job.

Both officials provided critical information about Trump during public hearings, with Sondland saying the president sought a "quid pro quo" with Ukraine's leader and Vindman criticizing Trump's conduct during a July 25th phone call with Ukrainian President Volodymyr Zelenskiy as "improper."

The moves came hours after Trump was asked about his press secretary's comments that his opponents should pay a price. “Well, you’ll see," the president told reporters.

Sondland said in a statement Friday night, "I was advised today that the President intends to recall me effective immediately as United States Ambassador to the European Union."

"I am grateful to President Trump for having given me the opportunity to serve, to Secretary Pompeo for his consistent support, and to the exceptional and dedicated professionals at the U.S. Mission to the European Union," Sondland said.

Democrats denounced the firings as retaliatory.

Sen. Bob Menendez, D-N.J., the ranking member on the Senate Foreign Relations Committee, called the dismissals "clear political retaliation, the likes of which is seen only in authoritarian countries around the world."

"Those who suggested President Trump's behavior would improve following his impeachment have been proven wrong. I sincerely hope that all members of Congress condemn this latest reprehensible, yet sadly predictable conduct by President Trump," Menendez said.

Sondland, a hotel magnate who was named E.U. ambassador after donating $1 million to Trump’s inauguration, testified twice before the House impeachment inquiry, and recalled more incriminating information in his second, and public, appearance Nov. 20.

His assertion that "everyone was in the loop" on the president's plan to get dirt on former Vice President Joe Biden and his son Hunter featured prominently in the president’s Senate impeachment trial.

He said he knew that House members have asked "was there a quid pro quo," adding that when it comes to the White House meeting sought by Ukraine's leader, "the answer is yes."

Sondland also acknowledged having spoken to Trump on the phone the day after the president spoke to Ukrainian President Volodymyr Zelenskiy, and assuring him the Ukrainians were ready to commit to the "investigations" Trump wanted.

Another witness had testified that Sondland told Trump that Zelenskiy will do "anything you ask him to” because he "loves your ass."

"That sounds like something I would say," Sondland laughed. "That's how President Trump and I communicate. A lot of four-letter words. In this case, three-letter."

After Sondland's testimony, Trump distanced himself from the diplomat.

"I don't know him very well. I have not spoken to him much. This is not a man I know well. He seems like a nice guy though," he told reporters.

Vindman, the top Ukraine expert on the National Security Council who testified during the House impeachment inquiry, was ousted from his job and "escorted out of the White House" on Friday afternoon, his lawyer said. Vindman's twin brother, who also worked for the NSC, was also removed from his post.

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"LTC Vindman was asked to leave for telling the truth," his attorney, David Pressman, said in a statement. "The truth has cost LTC Alexander Vindman his job, his career and his privacy."

Pressman added that "the most powerful man in the world — buoyed by the silent, the pliable, and the complicit" had "decided to exact revenge" on Vindman, who was removed from his job at the NSC.

Vindman's twin, Yevgeny Vindman, was also escorted from the White House on Friday, a source familiar with the matter confirmed to NBC News. He was working as a senior lawyer and ethics official for the NSC and did not testify in the impeachment inquiry. The New York Times first reported that Vindman's brother, who's also an Army lieutenant colonel, was fired and removed at the same time.

The twin, "a decorated Iraq War veteran, was escorted off of the grounds of the White House, suddenly and with no explanation, despite over two decades of loyal service to this country. He deeply regrets that he will not be able to continue his service at the White House," Pressman said.

An Army spokesperson told NBC News, "We can confirm that both Lt. Cols. Vindman have been reassigned to the Department of the Army. Out of respect for their privacy, we will not be providing any further information at this time."

Alexander Vindman had, as part of his duties, listened in on the July 25 call between Trump and Zelenskiy that was at the heart of the impeachment inquiry.

The Washington Post first reported on efforts by the White House to push out Vindman, citing two people familiar with the move, as part of a broader targeting by Trump of perceived enemies in the wake of his acquittal in the Senate after a two-week trial. The Post reported that Vindman would be reassigned to a job at the Defense Department.

Responding to questions about reports of Vindman's imminent ouster Friday, Trump said, "Well, I'm not happy with him. Do you think I'm supposed to be happy with him? I'm not. They'll make that decision. You'll be hearing — they'll make a decision."

National Security Council spokesman John Ullyot said, "We do not comment on personnel matters."

After Sondland's removal was announced, Trump's son Donald Trump Jr. tweeted a message to the lead House prosecutor in the impeachment case. "Allow me a moment to thank — and this may be a bit of a surprise — Adam Schiff. Were it not for his crack investigation skills, @realDonaldTrump might have had a tougher time unearthing who all needed to be fired," Trump Jr. tweeted.

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Democrats slammed the Vindman move.

"Lt. Col. Vindman honored his oath and did his duty when he came forward about President Trump’s abuse of power. He deserves the thanks and respect of the American people. But we know that this president believes the only loyalty that matters is loyalty to him personally," Rep. Eliot Engel, chair of the House Committee on Foreign Affairs, said.

"This is shameful, of course. But this is also what we should now expect from an impeached president whose party has decided he is above the law and accountable to no one. Any senator who voted to keep Trump in office thinking he has learned his lesson must answer for this and for whatever parade of abuses we see in the future."

Senate Minority Leader Chuck Schumer, D-N.Y., said, "This action is not a sign of strength. It only shows President Trump's weakness."

Sen. Kamala Harris, D-Calif., tweeted, "It's clear Trump is sending a signal to anyone who dares to come forward with the truth. Lt. Col. Vindman showed devotion to duty, something this lawless president knows nothing about."

Republican Rep. Paul Gosar of Arizona, meanwhile, celebrated the move on Twitter. "Good riddance," he wrote, linking to a tweet about Vindman's dismissal.

Vindman, who testified before the House in November under subpoena, told members of Congress that he was "concerned" about what he'd heard on the call and that he felt it was "improper for the president of the United States to demand a foreign government investigate a U.S. citizen and political opponent."

Vindman also testified that the summary of the July 25 call was transferred to a private, more secure server “to avoid leaks” and to help “preserve the integrity of the transcript," although he added that he felt that the move was not intended as “nefarious.”

During his testimony, Vindman, an Army lieutenant colonel who received a Purple Heart after he was wounded by an improvised explosive device in Iraq in 2004, faced repeated character attacks from several Republicans on the House Intelligence Committee, including over his loyalty to the U.S.

Vindman was born in Kyiv, then part of the former Soviet Union, and fled with his family to the U.S. as a child.

Vindman and Sondland aren't the only administration officials involved in Trump's impeachment to have had their career trajectories altered.

The former U.S. ambassador to Ukraine, Marie Yovanovitch, a central figure in the impeachment investigation who was recalled from her post last year, retired from the State Department last month. And Bill Taylor, who had replaced Yovanovitch as the top U.S diplomat in Ukraine and who also testified during the impeachment inquiry, was recalled a week before his term was set to expire in January. Secretary of State Mike Pompeo was scheduled to visit Ukraine in early January, and "is not going to want to be in a photo with Taylor," a congressional aide said at the time.

Vindman, in his opening remarks at the start of his public appearance before Congress, offered a stunningly personal message about how his family had come to America for a better life and how escaping an authoritarian regime instilled in him and his brothers a sense of duty to serve in the U.S. military.

He said that he never expected to testify about the president’s actions but he did so out of a "sense of duty" and said he recognized that his actions "would not be tolerated in many places around the world."

“In Russia, my act of expressing my concerns to the chain of command in an official and private channel would have severe personal and professional repercussions, and offering public testimony involving the president would surely cost me my life,” he said.

Addressing his father, Vindman concluded his statement by saying that “you made the right decision 40 years ago to leave the Soviet Union and come here to the United States of America in search of a better life for our family.”

"Do not worry, I will be fine for telling the truth," Vindman said.

Hallie Jackson reported from Washington, D.C., and Adam Edelman from New York.