According to CNN, a member witnessed Romney, 75, telling fellow Republican Santos, 34, "You don't belong here," as the representative passed by him in the House of Representatives chamber prior to President Joe Biden's speech.
Romney later criticized Santos for taking a front aisle seat and "trying to shake hands" with Biden, 80, which Romney considered inappropriate "given the fact that he's under ethics investigation," he told reporters.
"He should be sitting in the back row and staying quiet instead of parading in front of the president and people coming into the room," he said.
The senator also called Santos "a sick puppy" for his lies, according to USA Today.
He told CNN he didn't hear if Santos responded at the time. Santos, however, did seemingly make a more public response, tweeting "Hey @MittRomney just a reminder that you will NEVER be PRESIDENT!" after the event.
Santos has come under fire as he has admitted to lying on his resumé, biography and about his finances. He confessed he did not attend New York University or Baruch College, and never worked at Citigroup or Goldman Sachs, as he previously stated.
Alejandra Villa Loarca/Newsday RM via Getty George Santos
Further lies included that he was Jewish (to which he later said he meant "Jew-ish") and that his mother was in the World Trade Center during the Sept. 11 terrorist attacks, though it was later discovered she wasn't even in the country at the time.
Sipa via AP Images George Santos
Santos is currently being investigated at the federal and local level, and by the House Ethics Committee. Fellow New York Republicans have joined the chorus of people calling for him to step down.
House Speaker Kevin McCarthy, however, has yet to ask the freshman congressman to resign — a move that has been criticized by his colleagues. Romney has previously expressed his disappointment in McCarthy's lack of action.
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"He shouldn't be in Congress, and they're going to go through the process and hopefully get him out," Romney said to reporters. "But he shouldn't be there and if he had any shame at all, he wouldn't be there."