GOP Utah Senate nominee Mitt Romney is walking a careful line when it comes to his stance on President Trump as he prepares for a primary on Tuesday against a conservative challenger.

Romney, who was once a stern Trump critic, has refrained from openly scrapping with the president since he declared his intent to run to replace outgoing Sen. Orrin Hatch, R-Utah.

While he is the overwhelming favorite, he was forced into a runoff after he failed to win 60 percent of delegates votes at the state's party convention.

Romney has a choppy history with the president. While Trump backed Romney's presidential run in 2012, Romney and Trump feuded publicly for over a year as Trump ran for the 2016 nomination. The height of Romney’s criticisms came in March 2016, when he blasted Trump in a major speech.

"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.

But polls suggest Romney will pull through the primary comfortably. A poll by The Salt Lake Tribune and the Hinckley Institute of Politics released this week showed Romney up 42 points over Kennedy. That poll also suggests Romney would beat Wilson 58-20 if the election were held today.

The Associated Press contributed to this report.