Mitt Romney, who is set to be sworn in to the Senate on Thursday, started his new year with a public condemnation of President Donald Trump, whom he criticized as lacking the character and principles the office requires in a Washington Post op-ed published Tuesday night.

“When he won the election, I hoped he would rise to the occasion,” Romney wrote. “But, on balance, his conduct over the past two years, particularly his actions this month, is evidence that the president has not risen to the mantle of the office.”

As could be expected, the president fired back Wednesday morning on Twitter. “Here we go with Mitt Romney, but so fast!” he wrote. “Question will be, is he a Flake? I hope not. Would much prefer that Mitt focus on Border Security and so many other things where he can be helpful. I won big, and he didn’t. He should be happy for all Republicans. Be a TEAM player & WIN!”

As Trump noted, some have speculated that Romney’s decision to speak out against the president’s style—while still praising many of his policies and his appointment of conservative judges and Cabinet members—may mean he is positioning himself as a replacement to Jeff Flake, the Republican who most often publicly criticized the president.

In 2016, Romney spoke openly against Trump, calling the candidate a “phony” and “fraud.” Trump also criticized Romney, calling him a “failed presidential candidate” who “choked like a dog.” After Trump’s election, Romney changed his tune, and while he still said he thought the president was not “a role model for my grandkids on the basis of his personal style,” he praised some of the president’s policy decisions and expressed interest in joining his administration. Trump also publicly supported Romney in his bid for the Senate.

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