Alarmed by President Trump and the shrinking faction of Republicans willing and able to stand up to him, Mitt Romney is seriously considering one more run for public office.

Sources close to Romney tell TPM that he’s leaning toward a run if Sen. Orrin Hatch (R-UT) decides to retire, a move that sources close to Hatch say is more likely than not. And they say that while Romney was initially not keen on running for the Senate, the retirements of Trump-critical Sens. Jeff Flake (R-AZ) and Bob Corker (R-TN) and John McCain’s (R-AZ) ongoing health problems have left a void he thinks desperately needs to be filled.

“There’s a demand for people like him in the Senate. We’re losing people like Jeff Flake, who’s a conservative but an independent voice, we’re losing Bob Corker. Now more than ever we need statesmen and people with integrity in public office, and Mitt Romney fits that description,” former Romney spokesman Ryan Williams told TPM, after emphasizing he hasn’t talked to Romney about the race.

Flake’s decision to retire this week means the GOP senator most fiercely critical of Trump won’t be around for much longer. But the other Arizona senator’s status is likely weighing more on Romney’s mind.

McCain is the only Republican left in Washington who can truly stand up to Trump in an effective way at this point. He has the gravitas and celebrity of a former presidential nominee that gives him a huge platform, and he was just reelected and doesn’t need to worry about the GOP base. McCain can’t be dismissed as too moderate like Sens. Susan Collins (R-ME) and Lisa Murkowski (R-AK), or as a conservative gadfly like Sens. Rand Paul (R-KY) and Ben Sasse (R-NE).

McCain’s bleak cancer prognosis has left his political future uncertain. That paired with the loss of Flake and Corker, as TPM noted earlier this week, means barely anyone in the GOP worth noting could be left to buck the White House after 2018. That would change if the 70-year-old Romney, who led the anti-Trump GOP resistance throughout the 2016 campaign, steps out of retirement to run for Senate.

“If Mitt Romney runs for Senate in Utah, then we might see the beginnings of a counter-insurgency, but until then it does look like a mopping up operation,” National Review writer and Trump critic David French said on MSNBC Friday evening.

Romney took it upon himself to lead the anti-Trump charge throughout the GOP primary, with many of his deputies following him to battle Trumpism. After giving a nationally televised speech where he called Trump a “fraud,” Romney led the last-gasp efforts to stop him in the primary, campaigning from Utah to Ohio to try to deprive Trump of the nomination, slamming him even after he’d sown it up, and refusing to endorse him in the general election.

“Through the calculated statements of its leader, Trumpism has become associated with racism, misogyny, bigotry, xenophobia, vulgarity and, most recently, threats and violence. I am repulsed by each and every one of these,” he wrote in a March Facebook post.

“I don’t think you could have a bigger cultural clash, and Romney would give that point of view a very powerful and eloquent voice in Washington,” Alex Castellanos, a former Romney adviser who first fought Trump’s nomination then helped him in the general election, told TPM. “Donald Trump has unified the Republican Party, he’s purged it of the non-Trump Republicans. McCain may unfortunately leave because of his health, Corker and Flake are on their way out. There’s a vacuum of powerful establishment Republican voices.”

Hatch, who is 83, is publicly rejecting the idea that he’s decided not to run for reelection. But sources close to Hatch’s office tell TPM that he’s leaning that way — and wants Romney to step up if and when he decides to retire after tax reform efforts conclude.

“Hatch has told Romney ‘I want you to replace me if I don’t run again,'” a source close to Hatch’s office told TPM on Friday.

Romney would be a formidable candidate in Utah, where he has an immense reservoir of goodwill and where the Mormon Church holds sway, especially in GOP politics. But he’d likely face a primary challenge from a more conservative foe, likely with the backing of former Trump chief strategist Steve Bannon.

“Mitt doesn’t have a clear shot. Utah politics are so convoluted right now. Obviously if he jumps in that’d clear the deck of a couple of mid-tier candidates in deference to the Thirteenth Apostle. But Mike Lee was on that third tier eight years ago and emerged out of nowhere,” a Utah GOP strategist, who declined to discuss the race on record until Hatch makes a formal decision, told TPM.

Sources who know Romney also say public service truly animates him — that’s why even after all his harsh criticism of Trump he was willing to be Trump’s secretary of state.

“To you or me, you’re sitting on a few hundred million dollars and 30-some grandkids … Why would you waste all that being in the Senate? But Romney, to his credit, he views his role as someone like John McCain. If John McCain, the conscience of the Senate, isn’t around much longer who fills that role? I think Romney thinks he can fill that role,” said the Utah Republican.

It’s far from certain that Mr. Romney will go to Washington. But if he does, that could shake up a city where anti-Trump Republicans are an endangered species. And it would put Romney directly back into the spotlight after years in the wilderness.

“I do think that Mitt’s career and political journey are unfinished,” said Castellanos.