President Trump took aim Friday at one of his most vocal Republican critics – Sen. Mitt Romney – calling the him a “RINO.”

The president’s Twitter swipe at the senator from Utah and 2012 GOP presidential nominee came hours after Romney called Trump’s efforts to reverse President-elect Joe Biden’s victory in this month’s election a “undemocratic action.”

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Fox News, the Associated Press and other news networks projected nearly two weeks ago that Biden would win enough electoral votes to defeat Trump and become president-elect. Trump has yet to concede, as he hopes that a spate of lawsuits he has filed, a couple of recounts in key states or blocking some crucial states from certifying the vote will reverse Biden’s victory.

This week the president has repeatedly tweeted that “I WON THE ELECTION,” has vowed that “I concede NOTHING!” while continuing to charge that there was “VOTER FRAUD ALL OVER THE COUNTRY!”

And the president on Friday afternoon is meeting at the White House with the top two Republicans in Michigan’s legislature, sparking speculation Trump’s hoping the state’s GOP-dominated legislature would appoint a friendly set of electors, which overturn Biden’s victory in the state.

Romney criticized Trump on Thursday night.

“Having failed to make even a plausible case of widespread fraud or conspiracy before any court of law, the President has now resorted to overt pressure on state and local officials to subvert the will of the people and overturn the election," Romney emphasized in a statement posted to Twitter. "It is difficult to imagine a worse, more undemocratic action by a sitting American President.”

Trump fired back on Friday morning, slamming Romney as a “RINO,”  which stands for Republican In Name Only, and suggesting that Romney "feels he got slaughtering by Obama 'fair and square.'"

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Trump for years has attacked Romney for “choking” in the days before the 2012 presidential election, when the then-GOP presidential nominee was neck and neck with President Obama in the polls before Obama won the election.

Romney lost to Obama by a bigger electoral count – 332 to 206 – than Biden’s 306 to 232 current lead in the Electoral College count over Trump. But Romney lost the popular vote to Obama by roughly 5 million votes. Biden currently holds a roughly 6 million vote lead over the president in the national popular vote.

Another Republican senator, Ben Sasse of Nebraska, also criticized the president on Thursday.

Sasse said in a statement that he is telling his constituents to look at what the Trump legal team court argues rather than what they’re saying in public.

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"Based on what I've read in their filings, when Trump campaign lawyers have stood before courts under oath, they have repeatedly refused to actually allege grand fraud -- because there are legal consequences for lying to judges," Sasse wrote in his statement. "President Trump lost Michigan by more than 100,000 votes, and the campaign and its allies have lost in or withdrawn from all five lawsuits in Michigan for being unable to produce any evidence."

But the criticism from Sasse and Romney, who was the only GOP senator to vote to convict the president during this year’s impeachment trial in the Senate, is not shared by most fellow Republicans in the chamber, who for now are standing with the president.