By Jon Keller

BOSTON (CBS) – Remember back in March of 2016 when Mitt Romney issued a desperate appeal to his fellow Republicans to stop Donald Trump’s march to the GOP nomination? “Donald Trump is a phony, a fraud,” he said. “His promises are as worthless as a degree from Trump University.”

With that as context, the low opinion of Trump expressed by Romney in a Washington Post op-ed article comes as no surprise.

“When he won the election, I hoped he would rise to the occasion,” he writes, concluding that “the president has not risen to the mantle of the office.” And as he prepares to take his Senate seat, Romney promises that “I will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions.”

Trump allies are outraged. Romney’s own niece, Ronna Romney McDaniel, chair of the Republican National Committee, cast him as an ally of Trump’s enemies and called his comments “disappointing.”

But the most significant backlash came from the president himself, who zeroed right in on the political implications of a new GOP senator sounding willing – even eager – to break from the party line.

“Question will be, is he a Flake?” tweeted the president in a reference to retiring Sen. Jeff Flake, a frequent Trump critic. “Be a team player & win!”

You’ll recall that Flake gave the Trump team fits over his reluctance to play ball and his support for Special Counsel Robert Mueller’s Russia probe.

But unlike Flake, Romney is just starting his Senate career and has support from Utah voters for his criticisms of the president. He could team with other GOP dissidents to erase the party’s three-vote edge in the Senate.

And if you think Romney’s not eyeing a primary challenge to Trump in 2020, you don’t know Mitt.

On paper, Romney could be the perfect Republican challenger, a conservative supportive of key Trump-era fiscal policies with a more moderate track record on health care and a much more “presidential” demeanor.

His criticisms this week are a wager that six months to a year from now, “Trump alternative” will be wise political branding.

Jon Keller