Orem, Utah (CNN) On the Senate campaign trail, a Utah voter tries to give Mitt Romney a handful of re-elect Donald Trump buttons.

"You can give them to me. I'm not going to put them on because I haven't decided who I'll support yet. It's a little early," he says with a thank you and a chuckle.

Take the Trump button but don't put it on — a simple moment that sums up a complex situation for the former Republican presidential nominee.

Romney, who harshly criticized Trump as a candidate but has since offered targeted praise of some of the President's policies, is now on the path to the US Senate, with a primary on Tuesday he is expected to clear and a general election he's heavily favored to win.

Once there, he will have to navigate the unique complexities of Trump's Washington as someone who shifts between being a booster of the President and one of his biggest Republican critics.

Romney is asked so often by Utah voters how he will handle this dynamic, he decided to write an op-ed Sunday in attempt to synthesize his nuanced position.

The gist: He will support the President where he agrees and speak out when he doesn't.

"I do not make this a daily commentary; I express contrary views only when I believe it is a matter of substantial significance," Romney wrote in the Salt Lake Tribune.

He quoted Martin Luther King Jr. on the danger of silence in the face of what one knows is unjust.

"I appreciate the argument made by those who believe we should stay silent, but I cannot subscribe to it. I know that any criticism may lessen the President's flexibility to enact policy with which I agree, but that end does not justify my silence in the face of things that matter," Romney added.

In these final days of Romney's primary race, that has certainly been the case on immigration. He has repeatedly condemned the administration's policy that led to the separation of undocumented families.

"The policy that lead to the separation of children from their families at the border was heartbreaking, heart-wrenching, and simply wrong" Romney said here, responding to a young voter's question about it.

Criticizing Trump is familiar ground for Romney.

In 2016 he was the GOP "NeverTrump" poster boy after delivering a blistering speech, calling Trump a "phony, a fraud" and worse. He changed his tune eight months later as President-elect Trump considered him for secretary of state.

Among the ranks of his own party, critics of the President are hard to come by. As former House Speaker John Boehner said recently , there is no Republican Party, there's a Trump party.

"l think the President has overwhelmingly shown that he has great support with the rank-and-file members of the Republican Party and I think that's a great tribute to his capacity to pull them together and get him to support them. That's something which you have to acknowledge," Romney said in an interview with CNN when asked about Boehner's comment.

He continued, "At the same time, if he's going to say something that's right and good for the country, I'll be all for it. If he says something I don't feel is right, well I'm going to feel obligated to speak my mind because if the captain of the team says something that you think is seriously wrong, why unless you say something, it suggests you're going along with something that might have been said.

"I'm hopeful that I'll have nothing but positive things to say in Washington if I get there, but I also feel I'm driven by my conscience and my sense of morality more than anything else."

The few sitting Republicans have spoken up against Trump, from Sens. Jeff Flake to Bob Corker, are retiring. Another, Romney's 2008 GOP presidential opponent turned friend John McCain, is home in Arizona battling brain cancer.

Is he looking to fill that void?

"I recognize that if you're in Washington and you're elected as a leader of our country, even though you're only one of 100 in the Senate, you need to speak out on things you care very deeply about," Romney told CNN.

On Tuesday, Romney did not criticize Trump on two of his more controversial policies: the travel ban upheld by the Supreme Court and trade wars that have led to businesses like Harley-Davidson moving some jobs overseas.

"You know I think the argument that this was a Muslim-oriented ban fell apart when one recognizes that what 90-95% of the Muslims in the world do not live in the five countries that were affected," he told CNN Tuesday. "India of course, Indonesia, Bangladesh and so forth, Pakistan, major Muslim nations, and they weren't affected by this, so I think it became increasingly clear that this was focused on those individuals that were coming from countries where there is not an appropriate background capacity to check on people."

And on Harley-Davidson, Romney said, "This is still a negotiation. We'll see where it ends up."

At 71, this two-time GOP presidential candidate thought his campaign days were behind him. Then Utah Sen. Orrin Hatch decided to retire and urged Romney to run for his seat.

"I did not expect to do this after I lost to Barack Obama. Did you know I lost? Yeah," he tells crowds.

He says it took some convincing, particularly from his wife, Ann.

"My good wife began to work on me, saying, 'You know what, you've got an obligation to run.' I said, 'Why's that?'" he tells the Utah crowd.

"She said, 'Well, you could do more for Utah and you could do more for our country. If you're the senator, given all the work you've done in the Republican Party, all the people you've campaigned with, all the people that you've helped, you'll be able to let Utah to keep punching above its weight. So, you need to get into this,'" he went on.

Unorthodox path to Utah Senate bid

Romney first ran for Senate a quarter century ago in Massachusetts, challenging the legendary Ted Kennedy and losing by 17 points. He ran for governor there in 2002 and won, signing a Massachusetts health care law that became a template for Obamacare.

Although he still has a Republican primary to win on Tuesday against GOP opponent Mike Kennedy, he is already touting his experience in a blue state as a plus in gridlocked Washington.

"My legislature was 87-89% Democrat. It was not lost on me that to get anything done, I had to work with Democrats," Romney says.

But it was a job in Utah that endeared Romney to locals: heading up the 2002 Olympic committee after a bribery scandal almost sunk the games here.

His GOP adversaries, who include state party chairman Rob Anderson, call Michigan-born Romney a carpetbagger.

But that's not an easy sell to Utah's large Mormon population.

As 2012 GOP nominee, Romney was the first major Mormon presidential candidate. His ancestors were Mormon Utah leaders, including great great grandfather Miles Romney, an architect who designed the St. George Tabernacle.

"I joke that I'm a five-generation Utah, but my dad and I took a little time off for a business career," Romney said on the trail.

His father, George Romney, was elected governor of Michigan three times, and also launched two failed White House bids.

A more natural fit in Utah

When Romney ran for President, he was known to make awkward statements that landed with a thud on the trail.

Sometimes the straight-laced Romney's humor was just plain corny.

But it plays better in Utah.

He warms up his crowd apologizing for the food he serves at his events: Hostess Twinkies.

"That's right. Those were built about two and a half years ago, but I'm told they're still good," Romney said.

"There is nothing about a Hostess Twinkie that has ever seen the inside of a cow," sensing the crowd liked it and eager to keep his Twinkie riff going.

"Utah is a natural fit for Romney," a longtime confidante tells CNN.

He drives himself around in his black pickup truck. He's smiling, upbeat, looking more relaxed than he had on the national campaign trail.

But what about in the US Senate? Under the best of circumstances, former governors used to having executive power find being one of 100 in the U.S. Senate frustrating.

"I spoke to one senator who said, 'you're going to hate it. It scared me to death," Romney told CNN with a laugh,

"But, you know, this is a time that's critical," he added.