The Utah senator faced boos from other attendees at the convention ahead of the vote to censure him for voting to convict Donald Trump during his second impeachment trial, for inciting the Capitol insurrection.
Booing is audible in footage from the Republican convention, with the senator forced to stop mid-way through a speech, reported CNN.
"Oh yeah, you can boo all you like, but I've been a Republican all my life,” Mr Romney told the audience. “My dad was a governor of Michigan, my dad worked for Republican candidates that he believed in. I worked for Republicans across the country and if you don't recall, I was the Republican nominee for President in 2012”.
The senator continued by saying: "Yeah, I understand I have a few folks who don't like me terribly much and I — I'm sorry about that. But I express my mind as I believe is right and I follow my conscience as I believe is right”.
Mr Romney was one of seven Republican senators who voted to convict Mr Trump for inciting the Capitol riot after a second impeachment trial of the former president in February.
A year earlier he had been the only Republican senator to vote to convict Mr Trump in his first impeachment, when the then-president was accused of abuse of power and obstruction of Congress over the Ukraine scandal.
Utah Republican delegate Don Guymon was responsible for Saturday’s vote to censure the state’s senator for his vote, and for “publicly criticising president Trump”.
Delegates voted 711-798, according to Utah Republican Party spokeswoman Lynda Cox.
Mr Guymon said afterwards that the close vote was an indication of a “turn of the tide” against Mr Romney, who was first elected to the senate in 2018, after running as the party’s 2012 presidential nominee.
The booing of Mr Romney is an indication of the ongoing split between pro-Trump and anti-Trump Republicans after the former president’s unconventional four years in office.
A delegate from Salt Lake City, Emily de Azavedo Brown, told the Associated Press: “If the point of all this is to let Mitt Romney know we’re displeased with him, trust me, he knows.”
She added: “Let’s not turn this into a Trump or no Trump thing. Are we a party of principle or a party of a person?”
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