Representative Conor Lamb was supposed to be a Democratic rising star — a Marine veteran, former prosecutor and Pennsylvania moderate who had won in Trump territory and swing suburbs alike. Scores of Democratic officials endorsed him in his run for Senate, eager to pick up a Republican-held open seat and have him roll into Washington next year to bridge the partisan chasm.

It hasn’t quite worked out that way.

Mr. Lamb now heads into the state’s Democratic primary on Tuesday on a much less competitive footing than he or his supporters had hoped. He trails by double digits in polling behind John Fetterman, the shorts-wearing lieutenant governor whose outsider image has resonated with the Democratic base.

Two distinct forces appear to have worked against Mr. Lamb: his campaign’s strategic missteps and his misfortune to be running at a time when Democrats, much like Republicans, are rejecting their party’s centrists.

The seeming meltdown for Mr. Lamb — whose initial victories in Western Pennsylvania had been a model for President Biden’s 2020 race — reflects a frustration among Democrats nationally with politicians who promise bipartisan accord, including Mr. Biden, and who have yielded meager results in Washington. It comes as the left sees a rising Republican extremism on voting rights and abortion. Some Democrats appear more eager to elect fighters than candidates who might be tempted, like party moderates, to block their priorities.

“I look at him as another Joe Manchin,” said Elen Snyder, a Democrat and member of Newtown Township’s board of supervisors in Bucks County, referring to Senator Joe Manchin III of West Virginia, the Democrat who has stymied the White House on many issues. Her local Democratic committee interviewed Mr. Lamb but declined to endorse him.

Democratic strategists in Pennsylvania said the Lamb campaign’s missteps included running the race as if Mr. Lamb were the front-runner, failing to aggressively attack Mr. Fetterman and focusing almost exclusively on the message that Mr. Lamb was the most electable Democrat, when base voters appeared to want someone more partisan. And they said the campaign placed too much emphasis on winning endorsements from the Democratic establishment, when voters seemed to show that they did not really care.

“He had rock star potential — their campaign flittered that away,” said Mike Mikus, a longtime Democratic operative in Pennsylvania and a Lamb supporter. “They ran a campaign that said, ‘Let’s stay above the fray. Everyone’s going to love it.’ But they were behind from the day he got in the race and ran the wrong campaign to close the gap.”

Several strategists said the Lamb campaign, with its aversion to going negative and emphasis on endorsements from Democrats statewide, seemed modeled on elections from decades past. One operative invoked Michael Dukakis, the 1988 Democratic presidential nominee who projected reserve and lacked a killer instinct.

Abby Nassif-Murphy, Mr. Lamb’s campaign manager, disputed such characterizations. She said Mr. Lamb entered the race as an underdog and grew support that was more substantial than “dubious polls” have suggested.

“In nine months, he’s built a broad, diverse coalition of union workers, African Americans, women, men, progressives, moderates, religious leaders, teachers, firefighters, nurses, construction workers — people from all parts of Pennsylvania and all parts of the Democratic Party,” Ms. Nassif-Murphy said in a statement.

Long a battleground represented by center-right or center-left statewide officials, Pennsylvania could host a matchup in the fall between far less consensus-minded candidates, especially since the leading Republicans have all professed loyalty to Donald Trump. Kathy Barnette, who has surged in the final days, has actively promoted conspiracy theories about the 2020 election.

Mr. Lamb, 37, a native of the Pittsburgh area, boasts of scores of endorsements, including from the mayors of Pittsburgh and Philadelphia, officials in the all-important Philadelphia suburbs and members of the state legislature. He has the backing of the Black Clergy of Philadelphia and Vicinity, and of labor unions. Multiple Philadelphia officials endorsed him, even though a third candidate in the race, Malcolm Kenyatta, is from the city.

Mr. Fetterman has made his lack of endorsements into a kind of badge of honor: He has long disdained glad-handing other elected officials and is an unpopular figure even in the statehouse, where he officially presides over the State Senate.

Still, his progressive politics — he was an early backer of Bernie Sanders — and iconoclastic style have made him well-liked by the party base and created an online fund-raising juggernaut. Mr. Fetterman’s approval with Democrats in the state was 67 percent in a recent Franklin & Marshall College Poll, compared with 46 percent for Mr. Lamb.

“Fetterman astutely ran a campaign focused on Democratic voters more than Democratic elites,” said J.J. Balaban, a Democratic strategist in the state.

Melinda Wedde, a 37-year-old yoga teacher who is a volunteer door-knocker for the Lamb campaign in the Pittsburgh suburbs, said it was too early to count him out. “He’s out there talking to voters every single day,” Ms. Wedde said. “I think a lot of people are still waiting to make decisions.”

One advantage for Mr. Lamb in winning the endorsements from Democratic officials is that when he visited a town or city far from home, local officials often pulled in a crowd to hear him.

“People can say ‘establishment officials’ all they want, but these people are the trusted people in their communities, who people elected, and they have to have some sort of favorability amongst the masses,’’ said State Representative Ryan Bizzarro, a Lamb supporter who escorted him on a trip to Erie County on Tuesday.

Still, Mr. Lamb’s electability argument, the core of his pitch to party leaders, seems to have left many rank-and-file voters unmoved. And his central-casting image may be working against him.

“I don’t think they’re reading the voters,” Laura Brodie, a high school history teacher, said of Democratic officials. After voting for establishment-endorsed candidates in the past, Ms. Brodie, who lives in Westmoreland County, said she and other Democrats were disappointed in the results; she mentioned the likely reversal of abortion rights by the Supreme Court.

Ms. Brodie cast her vote early for Mr. Fetterman. “He doesn’t look like a guy who walked out of an Ivy League school,” she said. “He’s going to connect with different kinds of voters.”

At a recent debate, Mr. Lamb said he knew how swing voters felt about many issues. “They’re looking for stability and caution in their leadership,” he said.

But that message, with its echoes of Mr. Biden’s 2020 campaign, may be outdated.

Brendan McPhillips, who ran the Biden campaign in Pennsylvania, said Democratic voters are looking for something different: candidates committed to fighting a Republican Party they view as lurching dangerously to the right. While both Mr. Lamb and Mr. Fetterman support eliminating the Senate filibuster to pass Democratic priorities, Mr. Fetterman has gone further by making a frequent target of Mr. Manchin, the Democrat who has blocked filibuster reform.

Mr. McPhillips said that caution was the wrong message for the Democratic base today. “The stakes could not be higher,” he said. “We’re running against literal fascists. We need to reach for bolder ideas.”

From the outset, Mr. Lamb’s fund-raising couldn’t compete with Mr. Fetterman’s appeal to a national cohort of small online donors. After launching its first TV ads in March, the Lamb campaign was forced to take them off the air for almost all of April, a crucial month when voters were tuning in to the race. The campaign returned to TV late in April, and during the first week of May it spent $543,600 for ads in Philadelphia and Pittsburgh, according to AdImpact, an ad-tracking firm.

Mr. Fetterman’s TV ads have run continuously since March. He spent $537,000 in the first week of May, reaching the state’s two major cities as well as Wilkes-Barre and Harrisburg.

Some strategists said Mr. Lamb should have been aggressively attacking Mr. Fetterman in his ads — especially over a 2013 incident when Mr. Fetterman, who was the mayor of the town of Braddock at the time, brandished a shotgun to stop an unarmed Black jogger.

Some Democrats have warned that the incident will figure in Republican attacks in the fall if Mr. Fetterman is the nominee, in what they described as an effort to lower Black turnout.

Mr. Fetterman has said he made a split-second decision after hearing what he thought was gunfire. He has refused to apologize or say he did anything wrong. His refusal to apologize was criticized by both Mr. Lamb and Mr. Kenyatta at the only debate featuring all three candidates.

But Mr. Lamb’s ads have been exclusively positive, promoting his own biography and calling him “our best chance to beat the Republicans this fall.”

“For the life of me, I’ll never understand why the jogger incident wasn’t on TV and in mailboxes,” said Mr. Mikus, who ran a Democratic Senate primary race in the state in 2016. “They should have been on offense, putting him on his heels, letting people know what his flaws are.”