The episode could upend the still-fluid Democratic race and prove to be a setback for Mr. Sanders, who has largely avoided scrutiny of his age and his health. And it will increase pressure on him and his rivals to release detailed medical records going into a primary in which Democratic voters are desperate to find a candidate capable of defeating President Trump.
Mr. Sanders, Mr. Biden and Elizabeth Warren, who is 70, have all said they would release their medical records before the first voting starts in February.
When Mr. Sanders returns to the campaign, he may find what was already his biggest challenge to be all the more difficult: finding new converts to his mission.
“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.
Throughout the primary, Mr. Sanders has aggressively tried to dispel concerns about whether his advancing age makes him the best messenger for the Democratic Party. He has pursued a blistering campaign schedule often characterized by multiple stops every day, and has been loathe to take days off from the trail.
The setback with his heart 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 — but he has proven unable to expand his base beyond those enthusiasts. In recent weeks, he had shaken up his Iowa and New Hampshire staffs in an effort to jump-start his candidacy in the first two nominating states, where he has fallen behind Mr. Biden and Ms. Warren.
Mr. Sanders’s campaign 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 in Las Vegas that is dedicated to victims of the city’s 2017 mass shooting and 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 spoke to a crowd he estimated at 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 as he sat.