Senator Mitt Romney predicted he would face blowback from US President Donald Trump after he decided to vote to convict the leader of his party on abuse of power in the impeachment trial.
Within hours, the attacks started to come in.
Mr Trump posted a minute-long video in which the voice-over accused the conservative politician of being a "secret asset" of the rival Democratic Party, while denouncing him as "slippery" and "stealthy".
"Posing as a Republican, he tried to infiltrate President Trump's administration as the secretary of state," the attack ad charged, referring to speculation in 2017 that the president was considering Senator Romney for his cabinet.
Senator Romney, a former governor of Massachusetts, ran as the Republican nominee for president in 2012, losing to Barack Obama, who was re-elected that year for a second term.
He is deeply conservative and has largely voted in line with Mr Trump's objectives in the Senate.
In an interview with Fox News just before he cast his vote against Mr Trump in the Senate, becoming the lone Republican to buck his party's line, Senator Romney said he knew he would face repercussions but insisted he was going with his conscience.
"I understand there's going to be enormous consequence," the senator from Utah told the network, adding "I don't have a choice in that regard".
Calling the vote the "most difficult decision I ever made," Senator Romney said: "I can't let personal considerations, if you will, overwhelm my conscience and overwhelm my oath to God".
Donald Trump Jr, the president son's, called for the senator to be expelled from the Republican Party.
President Trump remains hugely popular within the Republican Party membership, giving him tremendous influence, though his aura of invincibility was broken in the last election cycle, when his endorsements failed to push key candidates over the finish line.
