"General Michael Flynn, the 33 year war hero who has served with distinction, has not retained a good lawyer, he has retained a GREAT LAWYER, Sidney Powell. Best Wishes and Good Luck to them both!" Mr Trump tweeted.
In a string of subsequent posts the US president quoted a constitutional lawyer as saying Congress would be unable to impeach him as they "would be putting themselves above the law".
Mr Trump's comments triggered immediate claims that the president's team may be attempting to interfere with the trial of Mr Flynn, coming a week after the release of an audio recording of the president's lawyer John Dowd requesting a “heads up” about Mr Flynn's interactions with the Robert Mueller investigation into Russian interference.
Brian Klaas, a political scientist at University College London, said Mr Trump's tweet, "combined with the audio recording of Dowd’s call with Flynn, is a blatant signal to Flynn and could amount to obstruction of justice". Mr Trump has previously denied the charge.
On Wednesday it emerged that Mr Flynn, the former US national security adviser, had hired Sidney Powell, a fierce critic of Mr Mueller's Russia investigation and conservative commentator who frequently appears on Fox News, as his new attorney.
The move came about a week after Mr Flynn fired two lawyers who had represented him since before the Mueller probe started in May 2017, prompting speculation the retired general might be looking to withdraw his guilty plea over lying to the FBI about his contacts with Russia.
At around the same time, the US Justice Department released a transcript of a phone call from Mr Dowd to Rob Kelner, Mr Flynn‘s lawyer, in which Mr Dowd sought information about Mr Flynn’s discussions with Mr Mueller.
Mr Flynn pleaded guilty to lying to FBI agents about his 2016 conversations with Sergey Kislyak, then-Russian ambassador to the United States. He was fired by Mr Trump after just 24 days in the job for misleading Mike Pence, the vice president, about the extent of his conversations with Mr Kislyak.
Calls for Mr Trump to be impeached gained momentum overnight after he gave an interview in which he said he would listen if foreign powers offered him damaging information about a political rival, saying he might not call the FBI.
Elizabeth Warren and Bernie Sanders were among the senior Democrats calling for the impeachment process to begin, with the latter tweeting: "We have a president who thinks he is above the law. The House should immediately begin impeachment inquiries."
Nearly half of the more than 20 Democratic primary candidates are calling for the start of an impeachment inquiry.
In response, the president tweeted a quote from constitutional lawyer Alan Dershowitz, who suggested Mr Trump was insulated from the risks of impeachment.
He wrote: "Congress cannot Impeach President Trump (did nothing wrong) because if they did they would be putting themselves above the law. The Constitution provides criteria for Impeachment - treason, bribery, high crimes & misdemeanors. Unless there is compelling evidence, Impeachment is not Constitutionally Permissable.”
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