Mr Romney, who has criticised the US president in the past, said Mr Trump had “disgraced” the presidency and that it was no surprise Republican voters were turned away by allegations that the 2020 presidential race was “rigged”.
"It turns out that telling the voters that the election is rigged is not a great way to turn out your voters,” Mr Romney was reported as saying, landing blame on the outgoing president.
“President Trump has disrespected the American voters, has dishonoured the election system and has disgraced the office of the presidency,” the Republican continued.
Political commentators on Wednesday suggested that the president’s allegations about a rigged election were to blame for lower than anticipated turnouts among Republicans in Georgia.
Senator David Perdue was headed toward a loss against Democrat Jon Ossoff, and senator Kelly Loeffler was projected to have lost against Democrat Raphael Warnock. Both challengers have since made victory speeches.
The results suggest that Republicans will lose control over the US Senate, which is due to vote to confirm Joe Biden as president-elect in a joint session of Congress, along with the House of Representatives.
Mr Romney said ahead of the vote that “I'm confident we'll proceed as the Constitution demands and tell our supporters the truth, whether or not they want to hear it.”
Alongside other Republicans in the Senate on Wednesday, Mr Romney has already ruled-out rejecting Mr Biden’s win during the joint session of Congress.
Mr Trump has leaned on Republican lawmakers and vice president Mike Pence - who is constitutionally bound to preside over the session - to reject the process, which could last into Thursday.
In a statement on Sunday, Mr Romney said Mr Trump and his allies in the Republic party were caught up in an “egregious ploy” that "dangerously threatens our Democratic Republic."
“I could never have imagined seeing these things in the greatest democracy in the world," Mr Romney added. "Has ambition so eclipsed principle?”
Meanwhile, Trump supporters filmed themselves harassing Romney as he flew to DC from Utah on Tuesday. ‘Why aren’t you supporting Trump’, the female protester asked the politician.
Later as he boarded the plane fellow passengers could be heard chanting ‘traitor’.