Just don't expect him to disagree with the Republican Party too much if he wins Utah's Senate election in November. He didn't even mention Trump when he announced his plan Friday to run for the Senate seat vacated by Sen. Orrin Hatch.
Romney, a Mormon who is popular in Utah, has a strong chance of becoming a senator, even if his previous experience in public office was in Massachusetts. The head of the Utah Republican Party and Democrat Jenny Wilson, who is running for Senate, have already criticized him as an outsider.
The 2012 GOP presidential nominee would boast a national profile rarely seen from a first-term senator. The conservative Romney would likely agree with Trump on most issues, such as the GOP tax law passed late last year. Policies over which he could clash with Trump include national security, relations with Russia and immigration.
"Utah welcomes legal immigrants from around the world. Washington sends immigrants a message of exclusion," Romney said in a video announcing his campaign Friday.
The frequent Trump critic could also use his influence to criticize the president's divisive statements or work on bipartisan deals, according to two former Romney aides. But Romney would likely have to hold back some criticism of Trump, said Kevin Madden, who was a spokesman for Romney's 2008 and 2012 presidential campaigns.
Trump remains highly popular among Republicans and Romney would be only one voice among 100 senators.
"People are going to have to manage expectations about Romney serving as the premier or main counterbalance to Trump inside the party. A lot of people think that's going to happen automatically," said Madden, a partner at public affairs consulting firm Hamilton Place Strategies.
Ryan Williams, another spokesman for Romney's 2012 campaign, added: "He's not going to be a daily critic of the president. But when there's something that rises to the level of where he feels the need to speak out, he'll do it."
Romney would likely have a different relationship with the president than the outgoing Hatch, who has praised Trump in recent months. After the tax plan's passage in December, the senator called Trump "one heck of a leader." Trump reportedly begged the 83-year-old Hatch to run for re-election one more time.
CNBC could not immediately reach Romney for comment.