Updated 4:59 p.m. ET
Picking a fight with the leader of his party the day before being sworn in, former GOP presidential candidate and incoming freshman Sen. 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 appeared on CNN later Wednesday and laid out the role he will play as a sometimes-Trump critic, if necessary.
"I don't intend to be a daily commentator," Romney said, noting he won't always seek out cameras in the Senate, but "if it's a matter of great significance, I will speak out."
Romney cited three instances in Trump's first two years as president in which he disapproved of how the president conducted himself: his response to the racist violence in Charlottesville, Va.; his support of Roy Moore, the Alabama Senate candidate who was accused of sexual misconduct with teenage girls while he was an adult; and Trump's continued vitriolic attacks on the media.
At times, Romney's essay in The Washington Post 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."
But Romney also ruled out a 2020 bid. "No," he told CNN's Jake Tapper. "You may have heard, I ran before. I've had that experience. I acknowledge the president was successful."
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; he 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 on Twitter to Romney's op-ed, asking, "is he a Flake?" referring to outgoing Arizona Sen. 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 Republican National Committee Chairwoman Ronna McDaniel, who is Romney's niece. "For an incoming Republican freshman senator" to attack Trump, she tweeted, "is disappointing and unproductive."
Romney responded on CNN to his niece's critique, saying it was "more civil" than it might have been across the Thanksgiving dinner table. He called her a strong supporter of the president and noted that she was doing her job as chairman of the Republican Party.
Romney also wrote that "not all of the president's policies have been misguided," citing corporate tax cuts, reduced regulations and the appointment of conservative judges. Romney said he will support Trump policies that 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."