Picking a fight with the leader of his party on the eve of being sworn in, former GOP presidential candidate and incoming freshman Senator Mitt Romney, R-Utah, writes that President Trump "has not risen to the mantle of the office."

In an op-ed in Wednesday's Washington Post, Romney said "Trump's words and actions have caused dismay around the world."

Romney opposed Trump's candidacy in 2016, calling him "a phony, a fraud," but was willing to join his administration and met with the president-elect over dinner, and was widely considered to be a candidate for Secretary of State. He was also endorsed by Trump in his Senate bid.

Not surprisingly, Trump quickly responded to Romney's criticism on Twitter, asking "is he a Flake," referring to outgoing Arizona Senator Jeff Flake, a prominent GOP critic of Trump. The President said Romney "should be happy for all Republicans," and should be "a TEAM player."

Other Republicans joined in Trump's criticism, including GOP Chairwoman Ronna McDaniel, who is Romney's niece. "For an incoming Republican freshman senator" to attack Trump, she tweeted, "is disappointing and unproductive."

At times, Romney's essay sounded like a campaign manifesto for a potential 2020 primary challenge to Trump. "The world needs American leadership," Romney wrote, "and it is in America's interest to provide it."

Romney wrote that "not all of the president's policies have been misguided," citing corporate tax cuts, reduced regulations and appointment of conservative judges. Romney said he will support Trump policies he believes are "in the best interests of the country and my state." But Romney said he will speak out against "significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions."

However Romney added a caveat: "I do not intend to comment on every tweet or fault."

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