Richard Painter, a law professor and former ethics lawyer in the George W Bush administration, called on all members of congress to “start the process” of impeachment or resign.
His comments came after Mr Trump accepted the Russian leader’s denial that the Kremlin meddled in the 2016 US presidential election, despite his own intelligence agencies’ conclusion to the contrary.
Mr Trump said Mr Putin had issued a “strong, impressive denial” and he saw "no reason" why Moscow should involve itself in an election the Russian president admitted he had wanted Mr Trump to win.
“Donald Trump trusts a hostile foreign dictator more than our own intelligence agencies,” Mr Painter, who is running for election to the Senate, tweeted. “This is uncharted territory. We must impeach now!”
He continued: “Trump’s conduct is treasonous. For that reason alone he must be removed now. Any member of the House or Senate who is unwilling to start the process needs to resign or be voted out. Top Republicans in Congress break with Trump over Putin comments.”
Elsewhere in Washington, the condemnation came in thick and fast from opponents and supporters alike. Former CIA director John Brennan called Mr Trump's actions "treasonous", while Republican Senator John McCain called it “one of the most disgraceful performances by an American president in memory.”
The Arizona Republican said the US-Russian summit in Helsinki was “a tragic mistake”, and that Mr Trump had proved not only unable, “but unwilling to stand up to Putin". He added the two men “seemed to be speaking from the same script” as Mr Trump made a “conscious choice to defend a tyrant”.
Joe Walsh, a former Republican congressman and prominent supporter of the Trump agenda, said the president was a "traitor" and called on all "decent" Americans to abandon their support.
Fox & Friends host Brian Kilmeade, usually a staunch defender of the president, said a tweet by Mr Trump that blamed "many years of US foolishness" for poor US-Russia relations was "ridiculous".
“That’s by far the most ridiculous tweet of late, and that is insulting to past administrations," he said. "He can’t be saying that going into the Russian summit.”
Newt Gingrich, former House speaker and consultant for the Trump campaign, said the president "must clarify his statements in Helsinki on our intelligence system and Putin.
"It is the most serious mistake of his presidency and must be corrected – immediately," he added.
Mr Trump later said he had “great confidence in my intelligence people” but insisted he would continue to push for Washington and Moscow to “get along”.
"I also recognise that in order to build a brighter future, we cannot exclusively focus on the past," Mr Trump wrote. "As the world’s two largest nuclear powers, we must get along!"
Chuck Schumer, minority leader in the Senate, said Mr Trump's performance in Helsinki showed he was "putting himself over our country" and "weakening our defences and those of our allies".
"A single, ominous question now hangs over the White House," he said in a statement. "What could possibly cause President Trump to put the interests of Russia over those of the United States?
"Millions of Americans will continue to wonder if the only possible explanation for this dangerous behaviour is the possibility that President Putin holds damaging information over President Trump."
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