Sanders thanked well-wishers Wednesday and used the moment to draw attention to his signature universal-health-care proposal. “None of us know when a medical emergency might affect us. And no one should fear going bankrupt if it occurs. Medicare-for-all!” he tweeted.

Sen. Bernie Sanders (I-Vt.) campaigns at Dartmouth College in Hanover, N.H., on Sunday. (Cheryl Senter/AP)

The problem emerged at a critical moment for Sanders and the Democratic campaign. The senator had just announced he had raised an impressive $25.3 million in the year’s third quarter, giving him a boost at a time when rival Sen. Elizabeth Warren (Mass.) had been gaining ground. Now he’ll be off the campaign trail for an unknown period, with the next Democratic debate rapidly approaching on Oct. 15.

Age and health have loomed over the 2020 presidential race more than any such contest in recent history. The three leading candidates for the Democratic nomination are septuagenarians — Sanders, 78; former vice president Joe Biden, 76; and Warren 70 — who have faced scrutiny over their physical well-being.

The Democrats are vying for a chance to face President Trump, 73, who was the oldest person in history to be sworn in as president for the first time. Although Trump’s physicians have given him a clean bill of health — sometimes extravagantly — he pointedly enjoys fast food and avoids exercise, despite holding one of the highest-pressure jobs in the world.

While some older presidents, such as Ronald Reagan, have been seen as reassuring father figures, others, like John F. Kennedy and Barack Obama, have appealed to voters as youthful, energetic leaders. “I think age is an issue for some voters as a proxy for being able to do the job and a proxy for being an agent of change,” said Democratic strategist Jesse Ferguson.

Facing a younger crop of Democratic candidates running on generational change, Biden, Warren and Sanders have sought creative ways to project youthfulness and vigor. Sanders played in a summer softball game and reminds voters he used to be a competitive runner. Biden told reporters over the summer he rides a Peloton bike to keep fit. And Warren frequently jogs to the stage at her events.

But they have also had to deal with concerns from voters, questions from rival Democrats, provocations from Trump and, occasionally, injury or illness. Biden, whose gaffes, misstatements and sometimes meandering speeches have plagued his campaign, has absorbed the brunt of these broadsides.

At the first Democratic debate, then-candidate Rep. Eric Swalwell (D-Calif.) urged Biden to “pass the torch.” Biden snapped back, “I’m still holding onto that torch!”

In the September debate, former Housing and Urban Development secretary Julián Castro went after Biden in a way that many saw as a dig at his mental acuity. “Are you forgetting what you said two minutes ago?” Castro asked Biden during a heated exchange over health care.

After the debate, Sen. Cory Booker (D-N.J.) took his own swipe at Biden. “I think that we are at a tough point right now because there’s a lot of people who are concerned about Joe Biden’s ability to carry the ball all the way across the end line without fumbling,” Booker told CNN.

The next day, Biden promised to release his medical records “before there’s a first vote,” referring to the Iowa caucuses Feb. 3. He also tried to defuse the tension surrounding the subject, jokingly asking a reporter, “Man, you want to wrestle?”

That didn’t put the topic to rest, and Trump has been particularly persistent in needling Biden about his mental sharpness. On Wednesday, the president said Biden was “less smart now than he ever was.”

Sanders, whose gruff persona has been parodied in popular culture for years, with comedian Larry David portraying him as a grumpy old man on “Saturday Night Live,” has long sought to brush off questions about his age.

“I think you look at the totality of a candidate,” Sanders said recently on “The Late Show With Stephen Colbert,” responding to concerns about his age voiced by former president Jimmy Carter, himself 95. “I’m in good health and running a vigorous campaign.”

Sanders, Biden and Warren have all committed to releasing their health records before the first nominating contest, although none has done so yet.

An artery blockage such as Sanders’s typically has a good prognosis and does not normally require a long time to bounce back from, medical experts said.

“People recover very quickly,” said Steven Nissen, chief academic officer at Cleveland Clinic’s Heart & Vascular Institute. “Often they can be back to work in days, and certainly within weeks.”

Nissen said he does not think age was a big factor in Sanders’s case, but “clearly if you’re older, you do have other issues that may come up.”

Coronary arteries, which feed blood to the heart muscle, can become blocked over time, and as the artery narrows it reduces blood flow and can cause chest pains.

When a patient such as Sanders comes in complaining of chest pains, doctors usually perform an angiogram to diagnose the problem, said Elizabeth Klodas, a cardiologist in Minneapolis. That means inserting a small catheter that goes up to the heart area, where it can inject dye into the coronary arteries, allowing doctors to see them through X-ray.

When the chest pains struck Sanders on Tuesday, he was in the midst of a busy campaign swing that took him from New Hampshire to Nevada. He had planned to hopscotch across California in the coming days.

That’s consistent with Sanders’s breakneck campaign pace in recent months. Last Sunday he made three stops in New Hampshire, followed by four more Monday, then flew to Nevada for a grass-roots fundraiser Monday night.

That tempo contrasts sharply with Biden, who normally conducts one or two events a day.

But there have been signs that the aggressive clip has taken a toll on Sanders. The senator lost his voice during a trip to Denver in early September, and it was still raspy at a debate a few days later. His campaign canceled two days’ worth of events in South Carolina a week later so he could rest his vocal cords.

In March, Sanders received seven stitches on his head after what a spokeswoman described as an injury from the edge of a glass shower door. The injury did not prevent him from appearing at scheduled campaign events.

Injuries and illnesses, even those that are not severe, can present challenges for a campaign, and it’s not clear whether Sanders’s latest health issues will factor into how voters see him.

In 2000, former senator Bill Bradley (D-N.J.) had to reassure voters his irregular heartbeat was not a significant problem. When former senator Paul Tsongas of Massachusetts sought the Democratic presidential nomination in 1992, he went to great lengths to show voters he had recovered from non-Hodgkin’s lymphoma, though he died five years later of problems related to the cancer.

Hillary Clinton and her team had to combat conspiracy theories about her health, many of them spread by Trump supporters, after she experienced a fainting spell in 2016.

It was unclear late Wednesday how long Sanders would be off the campaign trail or if the episode would change his approach to the race. A campaign spokesman declined to immediately provide more details about Sanders’s condition but said he was recovering at a Las Vegas hospital.

Before Wednesday’s news, the Sanders campaign had been planning to plow ahead aggressively in the key early nominating states and to air its first ad.

But Wednesday the campaign abruptly put off plans for a $1.3 million investment to air the ad in Iowa. “It’s just a postponement,” said campaign spokesman Bill Neidhardt. He declined to elaborate further or explain the reasoning behind the delay.

The campaign had touted the commercial as recently as Tuesday night, presenting it as part of a larger strategy to perform well in the early states. Campaign manager Faiz Shakir had told supporters on a call that the campaign would be “up on the air aggressively over the next few weeks leading up to the debate on October 15.”

Sanders’s Democratic primary opponents, meanwhile, wished him a swift recovery Wednesday. Warren said she texted and called Sanders but had not heard back. “I’m sure Bernie will be back on the trail soon. He’s tough,” she said.

Booker added, “Whatever you want to think about Bernie Sanders, I think most of America agrees that is one tough guy.”