Within hours, the attacks started to come in.
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 Romney for his cabinet.
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 Trump’s objectives in the Senate.
In an interview with Fox News just before he cast his vote against Trump in the Senate, becoming the lone Republican to buck his party’s line, Romney warned he would face repercussions but insisted he was going with his conscience.
“I understand there’s going to be an enormous consequence,” the senator from Utah told the network, adding, “and I don’t have a choice in that regard.”
Calling the vote the “most difficult decision I ever made,” Romney said: “I can’t let personal considerations, if you will, overwhelm my conscience and overwhelm my oath to God.”
Romney said he had suffered sleepless nights over the impeachment vote and worried about the implications for his wider family.
“I have spoken a good deal with my family because this will have a consequence.
“The blowback will have consequence, not just for me, but for my family, for my wife, for my sons, for my daughters-in-law, for my 24 grandkids,” said Romney.
“It’s going to get very lonely,” he admitted.
Donald Trump Jr, the president son’s, called for expelling Romney from the Republican Party.
“Mitt Romney is forever bitter,” he said, referring to the failed 2012 bid to be president.
“He was too weak to beat the Democrats then so he’s joining them now,” he added.
The president has often been called a bully by his rivals, and even some of his allies have been at the receiving end of his caustic tweets.
Trump remains hugely popular within the Republican Party membership, given 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.