“You know, we had zero for the last — zero people for the last 16 months. Can you believe it? We had 25 million come in under Biden.”

Former Representative Conor Lamb, whom Mr. Fetterman beat in the party’s 2022 primary race for Senate, said the video had “all the persuasiveness of a North Korean newscast.”

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.”

“Yes, I’m a Democrat. Why am I here talking to you today?” Mr. Fetterman said in a video filmed in front of a steel mill in Pennsylvania. He introduced himself as a “common-sense Democrat” who he said “is “always going to reject the extremes in socialism.”

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.”

Do they attend the party’s midterm convention in Dallas and risk alienating swing voters in their states by embracing President Trump at a time when many voters are frustrated with his second term? Or do they skip the gathering, potentially drawing Mr. Trump’s ire and losing out on a chance to fire up the Republican base?

The Republicans running in the most competitive Senate races — those considered tossups or leaning toward one party or the other by the nonpartisan Cook Political Report — have made different decisions ahead of the convention, which will unfold on Wednesday and Thursday and feature some of the biggest names in the party.

Mr. Trump’s remarks, scheduled for around 9 p.m. Eastern time, will coincide with the first game of the N.F.L. season, a Super Bowl rematch between the New England Patriots and the Seattle Seahawks, which is set to air on NBC. (On Thursday night, when Mr. Trump is scheduled to speak again, a San Francisco 49ers-Los Angeles Rams game will be streaming on Netflix.)

The football games are likely to siphon away millions of viewers who might otherwise watch the Republican gathering. Roughly 28 million people tuned in for last year’s N.F.L. opener.

The Republican gathering in Dallas this week, engineered by Mr. Trump as a party-boosting spectacle ahead of the midterm elections, is likely to struggle to match the mass audiences that the traditional nominating conventions achieve in presidential election years.

ABC, CBS and NBC say they have no plans to pre-empt prime time shows for Wednesday or Thursday’s festivities, as they typically do for the quadrennial party conventions. CNN and MS NOW said they would air their usual evening programs and pivot to Dallas if news warranted.

For those wanting to follow along in Dallas, countless streaming services and online outlets, including The New York Times, will carry a live feed. On cable, C-SPAN is planning full coverage, while simultaneously airing speeches by Democratic candidates on C-SPAN2.

Fox News is airing a special two-hour “Hannity” on Wednesday and Thursday beginning at 9 p.m. Several of the network’s other programs, including Bret Baier’s 6 p.m. newscast, have relocated to the American Airlines Center in Dallas for the event.

NewsNation is planning to air speeches by Mr. Trump and Vice President JD Vance. A spokesman for Newsmax said that the channel would air the most newsworthy portions — about 25 percent of the proceedings, he estimated.

In a community center, next to a towering pile of boxed, bottled and canned foods — Cheerios, veggie sticks and refried beans — Mr. Talarico railed against the “billionaire megadonors” he said would be “clinking their champagne glasses” at the Republican event.

“While we’re here serving our neighbors,” Mr. Talarico said, “they are toasting to their own corruption.”

His Republican opponent in the race, Attorney General Ken Paxton, was set to speak on Wednesday at the convention.

The food drive was one of two events that the Talarico campaign was holding in Dallas as a counterpoint to Republicans rallying behind President Trump in a downtown arena. The Texas Democratic Party was set to have their own event later on Wednesday near the convention, featuring a slate of other statewide candidates, including those running for attorney general and lieutenant governor. And several Democratic congressional candidates also planned an event on Thursday.

The Texas Democratic Party also paid for a moving billboard attacking Mr. Paxton to drive around Dallas on Wednesday, accusing him of “protecting pedos” because of his office’s handling of a child sex case. (Mr. Paxton’s campaign has said the case was resolved with the child’s interests in mind and has accused the Talarico campaign of “retraumatizing” the victim.)

Mr. Talarico also went on Fox News and offered what appeared to be a more moderate position on transgender athletes in sports than he has taken in the past.

“I oppose trans athletes playing in girls’ sports if it endangers safety or fairness,” he said in the interview. “Every sport has to be safe and fair. As a former public school teacher, I care a lot about our student athletes.”

As polls have shown a close race, with Mr. Talarico leading by a narrow margin in some surveys, Mr. Paxton has also begun talking about lowering prices, modulating a hard right message that has helped him win with Republican voters. Mr. Paxton issued a news release on Tuesday reminding Texans that, as attorney general, he had secured a settlement in June with producers over high prices and would soon distribute more than five million eggs to food pantries across Texas.

Frederick Douglass Haynes III, a pastor and a Democratic congressional candidate in Dallas, introduced Mr. Talarico, and mocked the Republican convention.

“It’s a pep rally, and I’m tripping because what are you cheering? " Mr. Haynes said. “Are you cheering the fact that we’re losing when it comes to the economy, we’re losing when it comes to health care, we’re losing a war that is unconstitutional, unnecessary, and unwinnable?”

Mr. Talarico’s campaign said that the goods had been collected by volunteers and weighed over 20,000 pounds — a number designed to contrast with the at the convention.

The groceries were intended for middle-class Texas families, the campaign said.

Among those in attendance was the former county judge for neighboring Tarrant County, Glen Whitley, a Republican who left office in 2022 and has grown disillusioned with his party’s rightward shift. He now supports Mr. Talarico.

“I truly believe that Ken Paxton is the most corrupt politician we have in Texas,” Mr. Whitley said in an interview. But, he added, Mr. Paxton “might have to take a second place, since Trump’s in Texas right now.”