Senator John Fetterman’s political loyalties have become an important question in Washington, with some of his fellow Democrats worrying that the Pennsylvanian could abandon the party just as it has an opportunity to claw back control of the Senate.

In fact, Mr. Fetterman’s regular critiques of Democrats have made him so popular among Republicans in his home state that they now view him more warmly than Pennsylvania’s Republican senator, David McCormick, according to a new poll from The New York Times, The Philadelphia Inquirer and Siena University.

Mr. Fetterman has become a fixture in the conservative news media, which regularly amplifies his criticisms of fellow Democrats as being too liberal. Turning himself into something of a modern-day political wrestling heel, he has emerged at key moments to performatively trash his erstwhile Democratic allies, much to the evident delight of home-state Republicans. He has denied, however, that he plans to leave the Democratic Party.

The Times/Inquirer/Siena poll, conducted last week, found that Mr. Fetterman had become far more popular with Republicans than with Democrats.

He was viewed favorably by 73 percent of self-identified Republicans, the poll found, compared with just 22 percent of Democrats. Only 63 percent of Republicans saw Mr. McCormick favorably, though he appears significantly less known than Mr. Fetterman: A much higher share of respondents said they had not heard of him or did not have an opinion.

Mr. Fetterman’s overall favorability rating of 46 percent is close to what it was the last time the Times/Inquirer/Siena poll tested it, in May 2024, when it was 48 percent.

But back then, 74 percent of Democrats viewed Mr. Fetterman favorably and 64 percent of Republicans viewed him unfavorably. That has now all but reversed, with 63 percent of Democrats viewing him negatively and most Republicans holding a positive view.

Mr. Fetterman, who was first elected in 2022 and would face a challenging re-election bid if he runs again in 2028, has said that he will “never” switch parties. And despite his criticism of Democrats, he has remained a reliable vote for most of the party’s priorities in the Senate.

But with both parties considering the possibility that Mr. Fetterman could emerge as a crucial swing vote in the Senate after the midterms, his loyalty to Democrats is being watched closely. Last month, he and Mr. McCormick established a joint fund-raising committee, a rare development for prominent lawmakers from opposing parties.

Mr. Fetterman’s fierce support of Israel during the war in Gaza is a primary source of tension with his fellow Democrats. Well after most of his Senate Democratic colleagues became highly critical of the Israeli government’s actions, he has remained steadfast in his backing of the country.

Some Pennsylvania Democrats, like Joseph Cook, expressed their frustration with the senator.

“It feels like a bait-and-switch with Fetterman,” said Mr. Cook, 48, an I.T. project manager from Villanova. “He’s presented himself as more amenable to mainstream Democratic policies, but now he’s very favorable to Trump.”

Mr. Cook, who said he had previously voted for Mr. Fetterman, said he now disagreed with the senator on Israel, the war with Iran and immigration. When asked if he would support Mr. Fetterman if the senator ran again, Mr. Cook gave a succinct “hell no.”

Nearly 60 percent of Democrats also thought Mr. Fetterman had been too supportive of Mr. Trump.

Pennsylvania Republicans, for their part, like Mr. Fetterman’s political shift.

“He is willing to meet in the middle more than most Democrats,” said Christine Mechtel, 38, a Republican in Washington County, a Pittsburgh suburb. “I just appreciate someone, anyone, putting their sword down and working together. That’s how our country should operate.”

Mr. Fetterman remains relatively popular with the state’s independent voters, who often swing general elections. In the poll, voters who identified as politically independent viewed Mr. Fetterman more positively than negatively. Independents were actually slightly more positive toward Mr. Fetterman now (49 percent) than they were in 2024 (45 percent).

Among Democrats, Mr. Fetterman’s views on Israel have been a particular source of concern. Nearly half of Democrats said he was “too supportive” of Israel. Among Democrats under 30 years old, 57 percent said he was too supportive.

That stands in stark contrast to another Pennsylvania Democrat who has been vocal — but not nearly as provocative — in his support for Israel, Gov. Josh Shapiro. Just 17 percent of Democrats said he had been too supportive of Israel.

“I think Fetterman has gone so far on Israel because he knows it’s an issue where he can get in front of Republican voters,” said Katie Wall, 40, a Democrat living in Philadelphia. “He keeps swinging in every direction on the issue.”

Ms. Wall, who works for a biotech company, said she did not share the same concern about Mr. Shapiro: “I do not agree with Josh Shapiro on Israel, but I respect that he’s been clear on his views about it from the beginning.”