(CNN) - The Republican opposition to President Donald Trump is poised to get a new champion: Mitt Romney.
Utah Sen. Orrin Hatch's announcement Tuesday that he would not seek an eighth term cleared the way for an all-but-certain run by the former Massachusetts governor and 2012 Republican presidential nominee, who now lives in Utah and is enormously popular among voters there.
But that popularity doesn't necessarily extend to Trump's White House, where establishment Republicans such as Romney are often viewed with deep skepticism. Aides to both men say they have a cordial relationship. And they exchanged pleasantries by phone after Trump's recent visit to Utah. But Romney has often been the first leading Republican to call out Trump for unpresidential conduct or divisive rhetoric and has split with the President over everything from the threat posed by Russia to whether Alabama should have sent Roy Moore to the Senate.
Romney's return to the national stage could open a new chapter in Trump's fraught relationship with Senate Republicans. Trump's GOP critics in the upper chamber, chiefly Bob Corker of Tennessee and Jeff Flake of Arizona, have announced their retirement and will leave Capitol Hill next January. But assuming he runs and wins in November, Romney would be in Washington for the duration of the Trump administration, a formidable politician with his own political base and no qualms about directly challenging the President's moral leadership, temperament and fitness to represent America around the world.
Polls suggest Romney would be easily elected in staunchly Republican Utah where Trump is far more unpopular than in many other red states. Reverence for Romney there and his role helping turn around the 2002 Salt Lake City Olympic games would essentially make him "bulletproof" (as one GOP strategist put it), giving him freedom to respond to Trump's attacks without electoral consequences.
"With the power of incumbency, he's probably virtually bulletproof once he's in there," said the GOP strategist, who requested anonymity to speak frankly about Romney's potential candidacy before he has actually announced. "He'll be able to speak his mind and call it like he sees it."
In the 2012 presidential race, Romney won Utah with 72.8% of the vote. In 2016, Trump did not even cross the 50% threshold, defeating Hillary Clinton in the state 45.5% to 27.5% (A third candidate, independent conservative and Utah native Evan McMullin won 21.5%).
Stylistically, the two men could not be more different. Romney, who is deliberate, diplomatic and steeped in policy, was often criticized as the 2012 Republican nominee for being too cautious. Trump -- as his wild tweet taunting Kim Jong Un demonstrated Tuesday night -- continues to be defined by his provocations and cavalier approach to serious matters like North Korea's nuclear arsenal.
Before they settled into an uneasy truce after the election -- when Trump even considered the 2012 Republican nominee as his secretary of state -- Romney delivered a remarkable March 2016 speech at the Hinckley Institute of Politics at the University of Utah calling Trump "a phony" and "a fraud."
At that point before the GOP had chosen its nominee, Romney said "dishonesty" was "Trump's trademark." He faulted him for reckless rhetoric about foreign affairs, and cautioned voters to take note of his personal attributes: "the bullying, the greed, the showing off, the misogyny, the third-grade theatrics."
Still, one Republican strategist close to the Romney camp pointed out that Romney and Trump generally agree in many policy areas. It's not clear, for instance, that Romney would have opposed the massive tax bill that cleared Congress last month and represented Trump's first major legislative achievement.
"I think people who are looking at this election and expecting it to be a referendum on Trump -- and here comes the white knight -- will be disappointed, including all of our brand-new Democratic superfans who love Mitt Romney" for challenging Trump, this adviser said.
There are, however, areas where the two disagree. Long before reports of Russia's election meddling, Romney called Russia the nation's greatest "geopolitical foe." In his books and writings, Romney has also expressed greater reverence for diplomacy.
Trump, who speaks most highly of colleagues who shower him with praise, clearly would have preferred for Hatch to seek an eighth term. In a tweet Tuesday, he offered congratulations to Hatch "on an absolutely incredible career."
"He has been a tremendous supporter, and I will never forget the (beyond kind) statements he has made about me as President. He is my friend and he will be greatly missed in the U.S. Senate!" Trump tweeted Tuesday.
In recent weeks, a number of Romney advisers were convinced that Hatch would run again, in part boosted by the tax bill's success and the President's complimentary words.
Hatch, a seven-term senator, told friends he was genuinely torn about the decision. That was particularly true as Hatch continued to raise money for his re-election campaign, including sending out invitations to a donor retreat this weekend in Park City.
During comments at the White House after the passage of the tax reform bill, Hatch praised Trump as "one heck of a leader," stating "we are going to make this the greatest presidency we've seen, not only in generations, but maybe ever."