Making his first presidential visit to Utah on Monday, Donald Trump went out of his way to praise the state’s octogenarian senior senator, Orrin Hatch. “You are a true fighter, Orrin,” Trump said before a crowd at the state capitol in Salt Lake City. “You meet fighters and you meet people you thought were fighters but they’re not so good at fighting. He’s a fighter. We hope you will continue to serve your state and your country in the Senate for a very long time to come.”

This might seem routine—a Republican president praising a veteran Republican lawmaker in his home state—but the political press knew something else was afoot. The 83-year-old Hatch, the longest-serving Republican senator in history, is eyeing retirement after seven terms. If he does so, former GOP presidential nominee and current Trump critic Mitt Romney may run for the seat. Polls show he’d be a shoo-in.

“Mitt’s a good man,” Trump insisted to reporters on Monday. But the Associated Press reported that “privately, Trump has signaled support for an effort to submarine Romney. Trump has vowed to try to block Romney, whom he views as a potential thorn in his side in the Senate, according to a White House official and an outside adviser who have discussed the possible bid with the president.”

But how much does Trump really have to fear from Romney in the Senate?

On the one hand, a Senator Romney could cause the president headaches, as Monday demonstrated. Hours after Trump endorsed alleged child molester Roy Moore in the Alabama Senate race, Romney reiterated his opposition to Moore’s candidacy:

Romney has also challenged Trump’s violation of American political norms and moral equivalency in Charlottesville. I’ve even argued that his ascension to the Senate could be good for the country. But National Review editor Rich Lowry sees an irony in Trump’s fear of Romney and embrace of Moore: “Romney would be a more reliable vote for the lion’s share of the Trump agenda than Roy Moore, who isn’t going to be a reliable vote on anything.” (Moore opposed the Republican plan to gut Obamacare earlier this year because it didn’t go far enough.) In a piece last month, Lowry noted Moore’s contempt for Senate Majority Leader Mitch McConnell, whom Trump desperately needs to secure any legislative accomplishments:

If Moore were in the Senate, he’d presumably be a reliable Republican vote like any other Alabama senator. The only difference is that he hates McConnell. Is that worth the reputational risk to the party of being associated with such a compromised figure? If there is a new Republican Senate leader in the next Congress, he sure as hell isn’t going to be a bomb-thrower (Senate leaders never are). So what’s the point?

National Review senior editor Jonah Goldberg was even more explicit back in September:

[Moore] will also almost surely say or do things that will encourage Republican senators in more moderate states to disassociate from Moore even when the actual policy position is right. Republican senators who need votes from independents and moderate Republican voters will not enjoy being linked to Moore in ads from Planned Parenthood and being asked by hostile reporters whether they agree with their Republican colleague’s views. In this and in myriad other ways, Moore will make it harder for Senate leadership to get things done — whether that leader is McConnell or someone else.

Lowry says Romney would be reliable on confirming conservative judges and advancing most GOP policy priorities, whereas Moore is a wildcard. Yet Lowry acknowledged that Romney wouldn’t have any personal loyalty to Trump, if the president faces potential impeachment. That’s why some prominent Democrats think Trump is right to fear Romney. “The fact that there’s more or less agreement on a number of issues is probably less important,” Democratic pollster Stan Greenberg told me. He believes that Romney, who famously called Russia our “number one geopolitical foe” in 2012, wouldn’t hesitate to challenge Trump on foreign policy.

Democratic strategist Tad Devine, who worked for Senator Ted Kennedy when Romney unsuccessfully challenged him in 1994, told me Romney “would be with Trump in terms of policy.” But he thinks the president may also be worried about his re-election. “If Mitt Romney became a member of the United States Senate, I think overnight he would become the number one challenger to Trump for the nomination,” he said. “I think Trump views him as a threat, as he should.” Under the right circumstances, Devine said, “I could see Romney standing up and leading people who want to impeach the president or push him out.... He’d be very quick to move to that posture.”

Surely some Democrats even welcome the prospect of a Senator Romney, assuming the alternative is Hatch or someone resembling the junior senator, Mike Lee. (Utah hasn’t elected a Democratic senator since the 1970s.) As he was leaving the Capitol on Monday night, Congressman Seth Moulton told me “the president should be scared” of Romney. “I don’t agree with Mitt Romney on everything,” the Massachusetts Democrat said, “but he is a leader and the president is not.”

But as evidenced by the current tax legislation—which Romney likely would have supported—there are limits to how much any GOP senator can contribute to the resistance. Democratic strategist Lis Smith described Romney as “NeverTrump before NeverTrump was cool,” and she agrees that his stature would allow him to take Trump on in a way most Republicans can’t. Yet Smith said Trump’s economic polices are “virtually indistinguishable” from those Romney ran on in 2012. “I see this purely as a matter of personality differences,” she told me. “Having a Mitt Romney in the Senate of course is better than having a Roy Moore in the Senate, but he’s still a Republican.... No Republican is going to save us in this fight.”

If nothing else, Democrats can take comfort that having Romney and Moore in the Senate would haunt Trump in different ways. “Romney is an enormous thorn in Trump’s side,” American Enterprise Institute scholar Norman Ornstein told me. “Moore is a constant reminder of Trump’s own sordid history.”