YORK, Pa. — John Fetterman’s latest ad boasts that his campaign has become a movement. Days before Pennsylvania’s primary on Tuesday, Mr. Fetterman is the front-runner for the state’s Democratic Senate nomination. But he insists that he is simply “doing my thing.”

“I’m just a dude that shows up and just talks about what I believe in, you know?” he said in an interview on Thursday in the deeply Republican county of York, standing across the street from The Holy Hound Taproom, a bar where he hosted a packed campaign event.

Just a dude.

Doing his thing.

That thing includes believing that “voting is kinda critical to democracy.” And pledging to “get good Democratic stuff done.” And referring to a potential Republican opponent as a “weirdo.”

Mr. Fetterman, the lieutenant governor of Pennsylvania, does not sound like any other leading politician in recent memory. And standing roughly 6-foot-8, with his uniform of basketball shorts and hoodies bearing occasional schmutz, he plainly does not look like one.

But as Tuesday approaches in a contest to determine the general-election contenders in one of the most closely divided states in the country, Mr. Fetterman is in a far stronger position than many party officials in Pennsylvania and Washington had anticipated. And if he wins the Democratic nomination, his candidacy will offer a clear test of whether politicians with vivid personal brands can overcome crushing national headwinds at a moment of intense political polarization.

To Mr. Fetterman’s many die-hard fans — who adore his family, can recite parts of his life story and sometimes credit Mr. Fetterman with renewing their interest in politics — his low-key, accessible style helps shape their perception of him as a relatable straight shooter.

“He seems such a down-to-earth guy,” said Kimberly Millhimes, 42, who said the Fetterman campaign stop at the bar was the first political event she could recall attending as an adult. Her assessment was repeated frequently in nearly two dozen interviews with Pennsylvania voters this week — that he is “real,” and not just a rote politician.

To his detractors and some skeptical voters, nominating Mr. Fetterman — a 2016 supporter of Senator Bernie Sanders who wore a sweatshirt to the White House Easter Egg Roll — could risk alienating voters in the more moderate suburbs who have increasingly embraced Democrats in the Trump era. He has also been dogged by a 2013 incident that could shape how Black voters across the state view him. When Mr. Fetterman was the mayor of Braddock, Pa., he brandished a shotgun to stop and detain an unarmed Black jogger, telling police he had heard gunshots. Some party strategists worry that the episode could become a liability in the general election in November.

Public polling in Pennsylvania has been sparse and there is theoretically still time for the Democratic race to be upended. But so far, there has been little evidence that any issues about his past or his persona have dented enthusiasm for Mr. Fetterman among many Democratic primary voters.

He ran unsuccessfully for Senate in 2016 but built a devoted following, and after defeating an incumbent to win his party’s nomination for lieutenant governor, he has been a visible statewide presence in office. He had attracted national attention as mayor of Braddock, a struggling former steel town he worked to help revitalize. But he drew new levels of notice as a cable-television fixture when Pennsylvania’s 2020 votes were being counted.

His campaign has had an overwhelming fund-raising advantage, a head-start on television advertising and an early entry into the race. Representative Conor Lamb — the polished moderate from Western Pennsylvania who has emerged as his closest rival — entered the race later and amassed notable institutional support, but has struggled to break through statewide or to effectively define Mr. Fetterman in negative terms.

On Thursday night, as he made the rounds at The Holy Hound, attendees clamored for selfies and offered him French fries or a beer — Mr. Fetterman, who often holds campaign events in breweries and bars, ruefully declined. “Hell yeah!” he replied to a young attendee who requested a photo. He thanked another supporter who had already voted for being “triple awesome.”

“I feel like I could get a beer with Fetterman and we’d hit it off,” said Robert Keebler, 45, a union worker in suburban Pittsburgh.

Mr. Fetterman is widely considered a progressive candidate who promotes issues like raising the minimum wage, legalizing marijuana and eliminating the filibuster, and fighting for voting rights, abortion rights and protections for L.G.B.T.Q. people.

“Some folks, you know, will be like, ‘The Democrats! The culture wars! What are you going to do?’ I’m like, ‘Bring it on!’” Mr. Fetterman said in York. “If you get your jollies or you get your voters excited by bullying gay and trans kids, you know, it’s time for a new line of work.”

But he is not embracing the left-wing mantle. When one attendee at a campaign event told him that he would be the “tallest Squad member” — the small group of left-wing members of Congress — he quickly responded that he “won’t be a Squad member, but I will be your next United States senator.”

Republicans, however, see opportunities to paint Mr. Fetterman as too left-wing for centrist suburbanites. Charlie Dent, a moderate Republican and former Pennsylvania congressman who voted for President Biden, said Mr. Fetterman’s “great challenge will be that he’s identified as a Bernie Sanders Democrat,” referring to the Vermont senator and democratic socialist.

And questions of general-election viability have been a central point of contention in the primary.

“Conor has spent this campaign uniting all types of Democrats in a way that can actually win in November,” Abby Nassif-Murphy, Mr. Lamb’s campaign manager, said in a statement. “John Fetterman has spent this campaign running away from his own far-left positions.”

Mr. Fetterman dismissed such concerns about his ability to connect with moderate swing voters, saying his polling shows him ahead in the suburbs.

“Say what you will about Bernie Sanders — at least he voted with Joe Biden,” Mr. Fetterman said, seeking to link his rival Mr. Lamb with Senator Joe Manchin III, the centrist Democrat from West Virginia who has opposed a range of Democratic priorities. Mr. Lamb’s campaign has rejected those comparisons, citing his own voting record.

Asked whether he rejected the Sanders Democrat label, he replied, “Of course I reject it. We just talked for 10 minutes about how we’re just running as a basic Democrat.”

His style is also a point of controversy. As she stood in a Whole Foods parking lot not far from Mr. Lamb’s suburban Pittsburgh hometown, Darlene Jicomelli said she liked Mr. Fetterman, but worried that his informal look could turn off some voters. She said she was undecided on whom to vote for.

“I think Conor Lamb has a better chance to win against the Republican only because maybe Fetterman is too — I think sometimes he might come off as not a polished person,” Ms. Jicomelli said.

Mr. Fetterman declined to directly answer when he started regularly wearing shorts and hoodies in professional settings and shrugged off the scrutiny. “I just dress to be comfortable,” he said.

He was far more forceful when asked about how he intended to energize Black voters should he win the nomination.

“Our support among Black voters is strong,” he said. “I’ve dedicated my entire career to working in the interests of marginalized Black communities.”

But Mr. Fetterman has faced sharp criticism for how he discussed the 2013 shotgun incident. He has refused to apologize or say he did anything wrong.

At the York event, Jeffrey Kirkland, the president of the board of directors of the local historic Black cemetery, said he was supporting Mr. Fetterman and praised the lieutenant governor for his work assisting the cemetery. But Mr. Kirkland also said he had told Mr. Fetterman that he wanted to increase the diverse appeal of the campaign, and that Mr. Fetterman was receptive. Mr. Kirkland noted that he was one of the few people of color in the crowded room.

“I think it would bode him well to try to do something to motivate the African American vote,” he said. “Turnout will be the key.”

Any winning Democratic coalition in Pennsylvania is thought to require maintaining recent suburban gains, fueling turnout in the big cities and reducing the margins by which Democrats lose rural counties where support for Donald Trump is strong.

Mr. Fetterman, a Harvard Kennedy School graduate whose appearance and demeanor conjure a blue-collar worker, makes a habit of holding events in counties that are home to persecuted Democrats grappling with stolen yard signs and sometimes-hostile neighbors.

A Fetterman event on Tuesday evening in conservative Westmoreland County took on the air of a party. Attendees sipped Bud Light and margaritas on an outdoor patio. Gisele Barreto Fetterman — the Second Lady of Pennsylvania who has embraced the acronym “S.L.O.P.” — interrupted her husband in the middle of his speech, handing him her drink while she regaled the crowd with a story about the geographic breadth of their support. And as Mr. Fetterman made the rounds, people told him they were delighted he had visited their county.

“We’re not just silly hillbillies,” said Gabrielle Keung, 27, who praised Mr. Fetterman for showing up and speaking bluntly. With Mr. Fetterman, she added, “There’s no wishy-washiness.”