The Vermont senator had two stents inserted in an artery after experiencing chest discomfort at an event. His campaign canceled his events and said he would be resting for several days.

Senator Bernie Sanders was recovering in a Las Vegas hospital on Wednesday after being treated for blockage of an artery, forcing him to cancel his events in the coming days and casting uncertainty over a candidacy already struggling to win new voters.

Mr. Sanders, 78, experienced “some chest discomfort” at an event Tuesday night, said Jeff Weaver, a longtime adviser; a medical evaluation found blockage in one artery, and two stents were inserted.

Mr. Sanders’s hospitalization is likely to intensify the focus on age in the Democratic race, even as much of the public debate has centered on policies like health care and immigration. The three leading Democratic candidates, as well as the president they are vying to challenge, are all in their 70s. One of those Democrats, Joseph R. Biden Jr., 76, has drawn his own age-related scrutiny because of his sometimes rambling discourses and uneven answers in debates.

Campaign aides did not provide any further information about Mr. Sanders’s condition and they have not said whether he had suffered a heart attack or whether he was just experiencing chest pain.

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Inserting stents into arteries is common in the United States — there are at least 600,000 such procedures a year, and perhaps up to one million. It is usually uncomplicated, and patients return home within a day or two.

“The prognosis is very good,” said Dr. Steven Nissen, chief academic officer of the Heart and Vascular Institute at the Cleveland Clinic. “Most patients are home the next day and back to work very quickly.” He said he did not see it as an impediment to returning to the campaign trail.

Still, it is unclear when Mr. Sanders will return to the campaign trail or whether he will be able to participate in the next Democratic debate, on Oct. 15 in Columbus, Ohio.

Have you or your loved ones had experience with stents? We want to hear how it may have affected your life and work. Please share your story in the comments.]

Mr. Sanders’s heart issue could prove to be a political problem for him, and it has the potential to change the dynamic of a still-fluid Democratic race. Mr. Sanders has largely avoided scrutiny of his age and his health. But he and his rivals will now be under increasing pressure to release detailed medical records as Democratic voters try to settle on the best candidate to take on President Trump, who is himself 73.

Mr. Sanders, Mr. Biden and Senator Elizabeth Warren, who is 70, have all said they would release their records before the first voting starts in February.

When Mr. Sanders returns to the campaign, he may find what was already his biggest challenge — finding new converts to his mission — to be even more difficult.

“I do think this makes it harder for him to persuade new supporters to come into his column because this will at least be in the back of people’s minds,” said Erik Smith, a longtime Democratic strategist.

The setback with his health also comes amid something of a political slump for Mr. Sanders in his second run for the presidency. He has continued to raise substantial amounts of money from his dedicated supporters — on Tuesday, his campaign celebrated an impressive third-quarter fund-raising haul of $25.3 million — and has remained among the top three contenders in the primary. But he has been unable to expand his base beyond those enthusiasts. In recent weeks, he shook up his staff in Iowa and New Hampshire, the first two nominating states, in an effort to jump-start his candidacy as Ms. Warren passed him in some polls.

Mr. Sanders’s campaign Wednesday said he began experiencing chest pain during a campaign event on Tuesday night in Las Vegas, where he had traveled for a series of appearances. He had visited an outdoor memorial dedicated to victims of the city’s 2017 mass shooting and had hosted a grass-roots fund-raiser at the Shiraz restaurant.

The restaurant’s owner, Raja Majid, said in a phone interview Wednesday that Mr. Sanders had spoken to a crowd of about 250 people. As Mr. Sanders began taking questions from the audience, he asked a staff member for a chair, an unusual request from a candidate who typically stands or paces onstage. “It’s been a long day here,” he said.

On Wednesday, Mr. Weaver spoke only in general terms about Mr. Sanders’s prognosis.

“Senator Sanders is conversing and in good spirits,” he said in the statement. “He will be resting up over the next few days. We are canceling his events and appearances until further notice, and we will continue to provide appropriate updates.”

Mr. Sanders was to travel to California and Iowa later this week.

Mr. Sanders’s allies quickly downplayed his procedure. RoseAnn DeMoro, a former leader of a nurse’s union and longtime Sanders surrogate, said “there are numerous presidents who have had heart problems and heart problems far worse” than what Mr. Sanders experienced Wednesday.

Yet many Democratic voters have expressed discomfort with nominating a candidate in their 70s. A Pew survey in May indicated that only 3 percent of Democratic voters believed the best age range for a president to be in was in their 70s. Forty-seven percent of those surveyed said they preferred a president in their 50s.

“I’m not an ageist, and I never have been, but as I get older I realize the limitations of getting older, and I can’t even begin to imagine the strain of being president of the United States,” said Steve Horner, 68, a retired special education teacher from Las Vegas who travels with a portable oxygen unit to help him breathe.

“I think it does matter,” added Mr. Horner, who said he had not picked a candidate yet.

Others acknowledged that age was an issue, but not the pre-eminent one.

“Would I prefer someone who is younger? Yes, but it’s not our top thing,” Elizabeth Bennett, 54, said at a gun safety forum in Las Vegas.

On the campaign trail, each of the three septuagenarian Democrats has sought to project an image of good health, with Ms. Warren making a point to jog to the stage of her campaign events and Mr. Biden running through parades.

The onetime captain of his high school track team, Mr. Sanders has done the same. He has pursued a blistering campaign schedule often characterized by multiple stops a day, and has been loath to take time off from the trail. He pitched in the softball game his campaign staged over the summer on Iowa’s “Field of Dreams,” and his aides have released other images of him playing catch or basketball.

Mr. Sanders’s events are usually high-energy affairs, where he regales enthusiastic crowds with his calls for “Medicare for all” and rails against the corporate and Washington elite.

In recent weeks, he has struggled with hoarseness, which forced him to cancel several events last month, but he then returned to the campaign trail. In March, he hit his head on the edge of a glass shower door, requiring seven stitches.
The Sanders campaign had planned to go on air with his first television ads of the campaign this week in Iowa, announcing a two-week $1.3 million buy on Tuesday. His campaign said on Wednesday that it was postponing those ads.The procedure Mr. Sanders had is generally not considered dangerous. Stents are tiny metal tube-shaped cages used to widen arteries in which blood flow has become impeded. They are inserted into coronary arteries when patients are suffering from angina, which is pain that results from clogged vessels, or heart attacks, in which blood flow is completely blocked.

Dr. Gilbert Tang, a heart surgeon at the Mount Sinai Hospital in New York, said that if Mr. Sanders’s heart was not damaged, he should make a full recovery. But he also sounded a note of caution, saying the risks depended on which artery was blocked: “We don’t know what the anatomy looks like and what kind of stent at what location,” he said.

During his first presidential run, Mr. Sanders released a letter from his doctor declaring that he was in “very good health.” The letter stated that Mr. Sanders had suffered several ailments during his life, including gout and diverticulitis. The letter also said Mr. Sanders had normal readings for blood pressure, pulse and blood count and that he had no history of cardiovascular disease.