Mitt Romney, who is in a runoff with Trump supporter Rep. Mike Kennedy for the GOP senate nomination in Utah, has tried to avoid criticizing the president during the primary campaign despite his history of dismissing the president at one time as a "fraud."

Romney has held his tongue and is now comfortably up in the polls over Kennedy. But most observers believe that Romney has the potential to lead anti-Trumpers in the senate  and it's only a matter of time before he reverts to criticizing the president.

The Hill:

"Here's what I know: Donald Trump is a phony, a fraud," Romney said. "His promises are as worthless as a degree from Trump University. He's playing members of the American public for suckers: He gets a free ride to the White House, and all we get is a lousy hat.”

Trump responded by calling Romney “a total joke” and a “disaster candidate.”

But after Trump won the election a few months later, Romney appeared to make peace with Trump and met with him as he was considered for secretary of state.

But just months later, Romney slammed Trump’s response to a white nationalist rally in Charlottesville, Virginia, in August 2017, saying he caused “racists to rejoice.”

Despite that, Trump endorsed Romney for his Senate bid while Romney has in turn refrained from explicitly criticizing the president. This month he even went so far as to predict that the GOP would rally around Trump and that he would be re-elected comfortably in 2020.

"I think President Trump will be renominated by my party easily and I think he'll be re-elected solidly," Romney told supporters in Utah.

Yet there have been signs that Romney still has the potential to be a thorn in Trump’s side if he wins in November in the model of outgoing Arizona Sen. Jeff Flake or fellow former presidential nominee Sen. John McCain, R-Ariz.

Romney has condemned the practice of separating families caught crossing the border illegally, saying it “puts America in a terrible light around the world.”

He also sent out a tweet this week agreeing with Laura Bush’s criticisms of the practice, saying “we need a more compassionate answer.” However, amid a firestorm of criticism aimed at Trump, Romney’s remarks have been relatively mild.

Romney may be particularly cautious of challenging Trump until he has put away the primary challenge from the right from state Rep. Mike Kennedy -- who is running as the homegrown, more conservative choice. Whoever goes through to the election will have a substantial advantage against Democrat Jenny Wilson.

Romney, the very image of establishment Republican, is too much the patrician to ever accept Trump. But Romney's dilemma is that the GOP is now Trump's party and if he wants to play, he's got to pay his dues. That includes biting his tongue and tempering his criticism of the president when Trump does or says something that Romney doesn't much like, as well as voting for Trump's legislative initiatives when the president cracks the whip.

I don't think Romney can do that. Trump is too coarse, too vulgar for an elitist like Romney to pay any mind. If he's elected in November, I doubt the honeymoon will last more than a month. And anti-Trump forces will have found a leader with the name recognition and respect in some quarters of the party to build an opposition to the president around.

Romney's prediction of a Trump win did not include a promise to support him for a second term. However feeble a 2020 challenge to Trump might be, Romney will probably support the most viable candidate.