“We gave you no tax on tips, no tax on overtime and no tax on Social Security for our great seniors.”

Mr. Lamb is one of several Pennsylvania Democrats who say they are weighing running for Senate in 2028, when Mr. Fetterman will be up for re-election. Mr. Lamb said party officials in the state had “already assumed Fetterman is a non-factor” in a future race.

Mr. Fetterman has become increasingly estranged from his fellow Democrats since Mr. Trump returned to office last year. While he has remained a reliable vote for Democratic causes in the Senate, Mr. Fetterman has served as a leading cheerleader for Mr. Trump’s war with Iran and for Israel during the conflict in Gaza.

In Washington, Mr. Fetterman long ago stopped participating in routine meetings with Senate Democrats, whom he infuriated by establishing a joint fund-raising committee with Senator David McCormick, the Pennsylvania Republican he introduced in his video on Wednesday.

Polling released last month from The New York Times, The Philadelphia Inquirer and Siena University found that Mr. Fetterman’s popularity with the state’s Democrats had cratered and that he was now far more popular with Republicans.

Mr. Fetterman has said he will “never” switch parties. He did not immediately respond to messages on Wednesday.

Representative Chris Deluzio, a Democrat who, like Mr. Fetterman and Mr. Lamb, hails from western Pennsylvania, said the senator was “making it harder” for state Democrats to win races in November by giving the impression that there is bipartisan support for Mr. Trump.

“If the Republicans want him, they can have him,” said Mr. Deluzio, who said he was also considering a Senate run in 2028. “This guy is not going to be a senator after 2028.”

And Lt. Gov. Austin Davis, who said he had been encouraged to think about running for Senate, accused Mr. Fetterman of misunderstanding the political moment.

“There is a time and place for bipartisanship, but this isn’t it,” he said. “If you willingly participate in this, you’re part of the problem.”

Mr. Fetterman introduced Senator David McCormick, Republican of Pennsylvania, and said that they would work together “and work with President Trump and defend the steel way of life.”

The appearance of a Democrat at a convention boosting the G.O.P., at a time when his party is claiming that they have made life harder and more expensive for Americans, was a boon for Republicans just weeks before crucial midterm elections in which they are battling to defend their slim congressional majorities.

And it undercut Democrats’ message as they work to focus the final crucial weeks of the midterm campaign on the president, whose approval ratings are sagging, and his unpopular policies. Instead, Mr. Fetterman’s brief comments underscored Republicans’ main argument in the campaign — that Democrats are extremists and socialists who cannot be trusted to govern.

It came amid recent speculation that Mr. Fetterman could switch parties, a move he has said he would “never” make but has appeared more likely as his breaks with his own party have become more frequent and bitter.

His decision to take part in Mr. Trump’s convention appeared designed for maximum shock value and served as a boost for Republicans’ otherwise lackluster gathering, which many of their most vulnerable incumbents and candidates have opted to skip as they work to gain some distance from an unpopular president.

At the height of Mr. Fetterman’s popularity with Democrats, he briefly harbored aspirations to run for president in 2028, according to former aides. Instead, frustrated with his own party and his standing within it, and aware of the attention he garners for criticizing it, he has sought allies on the other side of the political aisle.

In recent months, Mr. Fetterman has amped up his flirtations with Republicans, while still vowing that he would never actually switch parties. In July, he set up an unusual joint fund-raising committee with Mr. McCormick, with whom he has established a close, personal bond. And he has said that if his party officially turned against Israel, as some of its most progressive voices are urging, it would be the “red line” that would force him to leave.

But he has also been adamant that despite his disagreements with Democrats and his antipathy to party gatherings and caucus meetings, he had less in common with Republicans. In an interview in July, he told The New York Times he would “never” leave the party.

About 69 percent of likely Democratic voters in Pennsylvania view Mr. Fetterman negatively, according to a poll conducted last month by The New York Times, The Philadelphia Inquirer and Siena University. Most Republicans in his state, however, now hold a positive view of Mr. Fetterman, who has become a fixture on conservative news outlets bashing the policies of his own party.

The progressive Working Families Party announced last year it would recruit and support a primary opponent to challenge him in 2028.

As Mr. Fetterman has evolved into a lawmaker who barely resembles the man he presented to voters on the campaign trail in 2022 — a champion of the underdog who highlighted his ties to Senator Bernie Sanders, the democratic socialist from Vermont — several former staff members have grown incensed and vocal in saying he is abdicating his responsibilities.

A group of his disillusioned former staff members last week threatened to begin revealing private communications from Mr. Fetterman in which he expressed disdain for the day-to-day duties of being a senator. The former staff members, who made their announcement with an anonymous account on X, said they would begin releasing the material on Monday, when the Senate returned from a weekslong break.

Mr. Fetterman was elected in 2022, after he had a life-threatening stroke in the middle of the most competitive Senate race in the country.

Shortly after being sworn in, he checked himself in to Walter Reed National Military Medical Center to be treated for depression for six weeks, and appeared to make a remarkable recovery as he began speaking out about the importance of getting help when needed. But then in 2024, he began retreating from the job, rarely attending committee meetings, casting votes or holding town halls.

He has since expressed remorse for speaking openly about his mental health and claimed that the criticism he gets for missing votes and committee hearings is because of it.

“My doctor warned years ago: After it’s public that you are getting help for depression, people will weaponize that,” Mr. Fetterman told The New York Times in an interview last year. “Simple things are turned. That’s exactly what happened.”