PITTSBURGH — John Fetterman, the liberal lieutenant governor of Pennsylvania who won his state’s Democratic nomination for Senate on Tuesday, seemed to be cruising into what is shaping up as one of the most closely watched general elections in the country this fall. Then a stroke upended his plans.
Mr. Fetterman, the 6-foot-8, hoodie-wearing former mayor of Braddock, Pa., was not a favorite of the party establishment, but he electrified some progressive voters and a broader slice of the Democratic electorate that embraced his blunt-spoken, accessible style and welcomed his pledges to fight aggressively for party priorities in Washington.
“I’m just doing my thing,” he said in an interview last week. “I’m just a dude that shows up and just talks about what I believe in, you know?”
After canceling campaign events on Friday, Saturday and Sunday, Mr. Fetterman, 52, announced that he had had a stroke, was recovering and had not suffered any cognitive damage.
He was still in the hospital on Tuesday, when his campaign announced that he would undergo “a standard procedure to implant a pacemaker with a defibrillator,” adding, “It should be a short procedure that will help protect his heart and address the underlying cause of his stroke.”
It was unclear when he would be able to resume campaigning.
The health scare carried ramifications far beyond Pennsylvania. Democrats hold a majority in the Senate only by virtue of Vice President Kamala Harris’s tiebreaking vote. The party’s vulnerability had already been highlighted when Senator Ben Ray Luján of New Mexico suffered a stroke in January.
It also seemed jarring given Mr. Fetterman’s vigorous public image; he often was dressed as if he’d just left the gym.
“He may not look like a Senate candidate for New York or California, but he’s just fine for Pennsylvania,” said Ed Rendell, a Democratic former governor of the state. “He’s a very believable candidate for the working class.”
Mr. Fetterman, who holds a degree from the Harvard Kennedy School, served for 13 years as the mayor of Braddock, where he attracted attention for his efforts to revitalize a struggling steel town — and scrutiny over a 2013 episode in which he brandished a shotgun to stop an unarmed Black jogger, telling the police he had heard gunshots.
He ran unsuccessfully for the Senate in 2016 but gained an enthusiastic following, and went on to defeat an incumbent to win his party’s nomination for lieutenant governor in 2018. In that role, he maintained an active presence around the state, building name recognition that played an important role in his primary victory.
“He spent a lot of time in communities throughout the state,” said Senator Bob Casey of Pennsylvania, who did not take sides in the primary. “That’s something he’s been able to build on.”
Mr. Fetterman also made a name for himself in national progressive circles, receiving the endorsement of Senator Bernie Sanders of Vermont in 2018 after he backed Mr. Sanders’s 2016 presidential primary bid. And he gained fresh prominence with a broader range of voters as a cable-television fixture when Pennsylvania’s 2020 votes were being counted.
Several months later, he entered the Senate primary, the first major Democratic candidate to jump into the race, and cemented an overwhelming fund-raising advantage over his nearest rivals.
Mr. Fetterman campaigned on issues like raising the minimum wage, promoting criminal justice reform and supporting voting rights, abortion rights and protections for L.G.B.T.Q. people.
But he attracted just as much attention for his style, and some saw him as skilled at connecting with blue-collar voters. He favored basketball shorts and sweatshirts over button-downs and khakis and spent significant time campaigning in rural, working-class counties that had overwhelmingly voted for former President Donald J. Trump, hoping to improve Democratic margins in those areas.
Mr. Fetterman has repeatedly described himself as a progressive in the past, but in the Senate race he did not seek the left-wing mantle. He rejected a suggestion last week that he would join the “Squad,” a group of left-wing members of Congress, should he win.
Republicans and some Democrats, however, believe that he may be vulnerable to criticism that he is too far to the left for one of the most closely divided states in the nation, and especially for its more centrist suburbs, which have been vital to recent Democratic gains in the state.
“It’s good that Fetterman is going to these areas where Democrats have done poorly in these Republican counties, but I think his bigger challenge is going to be these suburban communities,” said former Representative Charlie Dent, a Pennsylvania Republican who said he had voted for President Biden.
Mr. Dent warned that Mr. Fetterman is seen by some as a “Bernie Sanders Democrat.”
The lieutenant governor lives in Braddock with his three children and his wife, Gisele Barreto Fetterman, the second lady of Pennsylvania, who has embraced the acronym “SLOP” and who, like Mr. Fetterman, has an active social media presence.
She insisted that he get checked out after feeling unwell on Friday, the Fettermans said.
His campaign said Monday that Mr. Fetterman had been “again evaluated by the neurologist who once again reiterated that John will make a full recovery.”
