Democratic leaders have sought to tamp down Donald Trump’s impeachment revenge tour after the president dismissed key witnesses in the investigation from their government posts, while attacking those who voted to convict him last week.

House Speaker Nancy Pelosi slammed Mr Trump after he fired Lieutenant Colonel Alexander Vindman, a former National Security Council director who testified about the president’s phone call with Ukraine, calling the move a “brazen act of retaliation”.

“President Trump is impeached forever,” the House speaker said in a statement. “The shameful firing of Colonel Vindman was a clear and brazen act of retaliation that showcases the President’s fear of the truth."

She added: “History will remember Lieutenant Colonel Vindman as an American hero.”

Senate Minority Leader Chuck Schumer also urged 74 inspectors general across the country to investigate all acts of retaliation against whistleblowers who have reported presidential misconduct, writing in a letter: “Without the courage of whistleblowers and the role of Inspectors General, the American people may never have known how the President abused his power in the Ukraine scandal.”

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The impeachment inquiry began after an anonymous whistleblower filed an official complaint about Mr Trump’s 25 July phone call with Ukrainian President Volodymyr Zelensky, in which he asked for the country to launch investigations into one of his 2020 political opponents, Joe Biden.

Several prominent members of Mr Trump’s inner circle have since shared posts online allegedly identifying the anonymous whistleblower — including the president’s son, Donald Trump Jr.

Mr Trump has meanwhile claimed victory in his impeachment after the Republican-led US Senate voted to acquit him in a 52-48 vote. In an incendiary conference held in the East Room after the vote, Mr Trump lambasted his apparent enemies and described the investigations that plagued his presidency as “bulls***” and a series of hoaxes.

He also reportedly demanded the ousting of Mr Vindman, who he called “very insubordinate” during the weekend after the former NSC adviser was escorted from the White House on Friday afternoon.

Gordon Sondland, the former US ambassador to the European Union who also testified against the president and confirmed a quid pro quo existed in the administration’s Ukraine dealings, was fired hours after Mr Vindman on Friday.

The president also spent recent days attacking those who voted for his conviction in the Senate, including Democrats in battleground states like West Virginia Senator Joe Manchin and Alabama Senator Doug Jones, as well as Utah Senator Mitt Romney, the sole Republican to break ranks with his party in support of impeachment.

Mr Trump retweeted posts during the weekend claiming the senator “stabbed Trump in the back” and wrote in his own tweet: “Romney hurt some very good Republican Senators, and he was wrong about the Impeachment Hoax. No clue!”

The Republican Party has appeared to follow suit when it comes to the president’s attacks against Mr Romney, who became the first senator in history to vote against a president in their own party during an impeachment trial. Mr Romney was banned from the upcoming Conservative Political Action Conference, a prominent annual political conference, as the chairman said he would “actually be afraid for [Mr Romney’s] physical safety” if he were to attend the event this year.

"We won’t credential him as a conservative,” CPAC chairman Matt Schlapp told Full Court Press. "I suppose if he wants to come as a non-conservative and debate an issue with us, maybe in the future we would have him come. This year, I’d actually be afraid for his physical safety, people are so mad at him."

He added: "The biggest problem we have with Mitt Romney is not that he’s just an individual following his political course. It’s the fact that he’s lied so continuously to conservatives. He’s a ‘use-em-and-lose-em’ kind of guy."