SALT LAKE CITY — One man is an island: Sen. Mitt Romney of Utah.

The 72-year-old former Republican presidential nominee has isolated himself from Republicans in the Senate, in his home state and across the country by occasionally — but strongly — criticizing President Donald Trump, including the president's efforts to enlist the aid of foreign governments to probe a leading political opponent.

"By all appearances, the President's brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling," Romney tweeted earlier this month.

In recent weeks, the senator's acts of rebellion against the commander in chief have been flagrant: from publicly confirming "Pierre Delecto" as the secret identity he used to counter Trump on Twitter to bashing Trump's Syria policy on the Senate floor to positioning himself on the front edge of any move by GOP lawmakers to break away and either censure the president or vote to remove him from office if the House follows through with impeachment.

While that House-side inquiry has put a heat lamp on Republican senators from states where voters aren't thrilled with the president's actions — particularly swing-state lawmakers who are up for re-election in 2020 — Romney's criticism of Trump hasn't prompted those colleagues to follow him into the political no-man's land of finding fault with both the president's conduct and the divisiveness of impeachment.

Rather, it has renewed speculation among GOP critics in Washington and in Utah that Romney has ulterior motives — jealousy, retribution, Oval Office ambition or some potent mix of all three. After all, Romney ran for president twice and lost before Trump won the job, and then Trump made him publicly audition for secretary of state before passing him over.

And there's reason for speculation that Romney might still have designs on a job his father ran for more than half a century ago. Even as he has angered Republican senators in Washington and activists at home, he is one of the few in the GOP with the national name-ID and fundraising prowess to mount a serious campaign for the presidency on an abbreviated timeline.

But Romney's chief of staff Matt Waldrip reiterated what the senator has said in the past: That's just not in the cards. "He has ruled it out," Waldrip said.

As for Democrats, they're happy enough to have a talking point that hurts Trump, but they tend to dismiss Romney as ineffectual because he has stopped short of calling for Trump's impeachment and removal from office.

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Even GOP voters in this heavily Republican, majority-Mormon state have expressed dismay with Romney amid his latest anti-Trump rhetoric.

President-elect Donald Trump sits at a table with former Massachusetts Gov. Mitt Romney and Reince Priebus, left, at Jean Georges Restaurant on Nov. 29, 2016 in New York. John Angelillo / UPI / Pool via Sipa USA

In other words, he's all by himself.

Even Romney acknowledges he doesn't have much influence with his fellow lawmakers.

"Nobody's said anything," he told NBC in a brief exchange outside the Senate chamber last week. "Everybody expresses their own views in the way they should. People don't talk to each other or criticize each other or praise each other."

But surely the senator with the screen-ready silver-flecked hair might be forgiven if he doesn't know that silence is the oldest form of communication in the world's most deliberative — and talkative — body.

Romney's lack of influence was on full display this past week. By Friday morning, one day after Sen. Lindsey Graham, R-S.C., introduced a Senate resolution to condemn the House impeachment inquiry, only three GOP senators, Susan Collins of Maine, Lisa Murkowski of Alaska and Romney, had not added their names in support.

Knowing how popular the president remains in the Republican Party — and how afraid GOP members of Congress are of drawing Trump's wrath — Romney was aware that he would be separating himself from the pack when he chose to speak out, as he did during the 2016 election, according to advisers who spoke to NBC on the condition of anonymity because they were not authorized to discuss the matter on the record.

But, they contend, it shouldn't be difficult for a senator to say it's wrong for a president to ask foreign countries to interfere in American elections. "He's staked out a position alone just by stating the obvious," said one of his advisers.

Donald Trump appears at a news conference to endorse Republican presidential candidate, former Massachusetts Gov. Mitt Romney with his wife Ann Romney, at the Trump International Hotel and Tower Las Vegas on Feb. 2, 2012. Ethan Miller / Getty Images file

As for impeachment, the advisers said, Romney is grappling with his concern that the entire process is bad for the country and will end up being even more divisive no matter the outcome, a worry that he shares with his wife, Ann Romney. It's too early to say where he will end up on the question of removing the president from office, other than that his current position (he says he's never spoken about impeachment ) would allow for him to go in either direction.

None of that means Romney's recent criticism of the president — including a stern Senate floor repudiation of Trump's Syria policy this month — has been ignored in Washington.

"He gets heard," said Sen. Richard Burr, R-N.C.

For many Republicans, they just don't like what reaches their ears.

Sen. Jim Inhofe, R-Okla., said he wishes Romney and Trump would keep their feelings about each other — Trump has called Romney a "pompous ass" on Twitter — to themselves.

"They don't like each other," Inhofe said, adding that he thinks Romney's take on Trump is based on personal issues rather than policy or the president's official actions.

"Oh, yeah, it is," he said. "Can't figure out why. But it is."

In Utah, many of the more than two dozen Republican voters interviewed by NBC News last week also expressed disapproval at Romney's digs at Trump and his generally receptive approach to the House investigation into the administration's Ukraine affair.

About Romney's shots at Trump, most had harsh words for their junior senator, with many accusing him of harboring feelings of resentment and envy stemming from his failed 2012 bid and the Trump State Department saga.

Dan Roehm, from Davis County, in Temple Square in Salt Lake City on Oct. 22, 2019. Kim Raff / for NBC News

"It feels an awful lot like a personal vendetta to me," said Dan Roehm, an engineer from conservative Davis County, just to the north of Salt Lake City. "Because other than that, I really don't see what Romney's issues with him could be. I think they agree on like 80 percent of policy issues."

"Romney is not representing us," added Roehm, who voted for Romney in 2018 but says he no longer supports him. "I've written him a letter letting him know I'm disgusted with how he's acted on impeachment. He's been small."

Diane Pankratz, a 76-year-old retiree from Tooele County in the northwest corner of the state, said, "Mitt is a good-hearted person. I just don't think he's very conservative. He's leaning toward the Democratic side on this, and I guess I don't understand why.”

"He has the right to say how he feels," added, noting that she is glad Romney is holding the president accountable. "And it's hard for people to say something that’s not popular. I do admire that. But we should support our president."