Utah Republican Senator Mitt Romney is facing a backlash from Republicans angered by his decision to vote against Donald Trump in his impeachment trial.
The failed 2012 presidential candidate made history on Wednesday as the first US senator to vote to remove a president from their own party.
Republican figures, including Mr Trump himself, have slammed him as a "sore loser" and a secret Democrat.
Mr Trump was acquitted in the Senate, despite Mr Romney's lone dissent vote.
In a speech at the White House on Thursday, President Trump called his opponents, including Democratic Speaker Nancy Pelosi, "vicious and mean".
"They took a phone call that was a totally appropriate call - I call it a perfect call, because it was - and they brought me to the final stages of impeachment," he said.
"But now we have that gorgeous word. I never thought a word would sound so good - it's called, 'total acquittal'."
Influential figures in conservative media have called on Republican rebel Mr Romney to step down from the party.
And earlier on Thursday morning, President Trump took a swipe at him when addressing the National Prayer Breakfast, criticising people who use their faith to justify "wrong" actions.
But Mr Romney's colleagues in the Senate have been careful not to be too critical.
Asked whether Mr Romney would be ostracised from his party, Senate Republican Leader Mitch McConnell told reporters: "We don't have any doghouses here. The most important vote is the next vote."
Mr Romney spoke on the Senate floor ahead of his vote on Wednesday to say "the president is guilty of an appalling abuse of public trust," and that what he "did was wrong, grievously wrong".
He spoke of his Mormon faith and "oath before God" that demanded that he vote for conviction.
After his vote to remove Mr Trump for abuse of power, President Trump tweeted a video that referred to Mr Romney as a "Democrat secret asset".
"Had failed presidential candidate @MittRomney devoted the same energy and anger to defeating a faltering Barack Obama as he sanctimoniously does to me, he could have won the election," he tweeted early Thursday morning.
At the National Prayer Breakfast later on Thursday, a cross-party event held annually in Washington DC, Mr Trump appeared to take a swipe at Mr Romney saying: "I don't like people that use their faith as justification for doing what they know is wrong.
"Nor do I like people that say 'I pray for you' when they know that's not so."
This has been interpreted as an attack on Democratic House Speaker Nancy Pelosi, who has said she prays for Mr Trump and was sat four seats from him as he questioned the sincerity of her faith.
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Reacting to Mr Trump's prayer speech, Mrs Pelosi told reporters: "You're impeached forever. You're never getting rid of that scar.
"And history will always record that you were impeached for undermining the security of our country, jeopardising the integrity of our elections and violating the Constitution of the United States."
Mr Trump will make an address to the nation regarding his impeachment later on Thursday.
Author Anne Coulter called Mr Romney a "useful idiot" for Democrats, adding that he is "now finished in national politics".
"Romney's speech proves he IS John McCain. Fine, Republicans, do the cowardly thing, but please stop demanding that we admire your courage."
Florida Senator Rick Scott tweeted that his colleague "is wrong". "His decision to buy into [Democratic Congressman] Adam Schiff's partisan charade is disappointing. And he will ultimately be judged by the voters of Utah."
Florida Republican Congressmen Matt Gaetz of Florida and Lee Zeldin of New York both called Mr Romney a "sore loser".
"Mitt Romney absolutely despises that Donald Trump was elected POTUS & he was not," tweeted Mr Zeldin.
"The sore loser mentality launched this sham impeachment & corruptly rigged & jammed it through the House. It looks like Schiff recruited himself a sore loser buddy on the GOP side to play along."
Mr Romney's own former campaign spokesman, Rick Gorka, said his old boss was "motivated by bitterness and jealousy".
Sam Nunberg, a former adviser to Trump's 2016 presidential campaign, rhetorically asked if Mr Romney was "making a last ditch effort to become" the Democratic 2020 nominee.
Radio host Mark Levin tweeted: "He'll be seen for what he is: a petty, self-promoting, NeverTrumper who has contributed to the Radical Democrats' assault on the Constitution."
Mr Trump's eldest son, Don Jr tweeted a vulgarity to refer to Mr Romney.
"He was too weak to beat the Democrats then so he's joining them now. He's now officially a member of the resistance & should be expelled" from his party, Mr Trump Jr added.
Mr Romney's own niece, Ronna McDaniel, who chairs the Republican National Committee, said in a statement: "This is not the first time I have disagreed with Mitt, and I imagine it will not be the last."
"The bottom line is President Trump did nothing wrong, and the Republican Party is more united than ever behind him."
Mr Trump was impeached by the House of Representatives in December for abuse of power and obstruction of Congress
He was accused of pressuring Ukraine to conduct two investigations for his own political gain and to the detriment of national security.
Democrats say he dangled two bargaining chips - $400m of military aid to Ukraine that already been allocated by Congress, and a White House meeting with Ukraine's new leader.
They said this political pressure on a vulnerable US ally amounts to an abuse of power.
But after the Democrat-controlled House of Representatives brought the third impeachment in US history, the Senate voted to acquit.
At the prayer breakfast, Mr Trump described his impeachment as a "terrible ordeal" perpetrated by "very dishonest and corrupt people" who "put themselves ahead of our great country".