The fraught debate over whether someone is too old to manage the rigours of the U.S. presidency has largely been overlooked during a Democratic primary that has put more emphasis on issues such as health care, immigration and gun control. That changed this week.

The hospitalization of 78-year-old Bernie Sanders to treat a blocked artery in his heart ensures that the question of how old is too old to be president moves to the forefront of the Democratic contest.

An uncomfortable fact for the party -- that three of its leading contenders are 70 or older -- will be impossible to ignore. If any of those candidates were to become the nominee, they wouldn't be able to use 73-year-old President Donald Trump's age against him. And perhaps most importantly to a party determined to defeat Trump, a septuagenarian nominee would have to work harder to present himself or herself as an agent of change, typically a key argument in any effort to defeat an incumbent president.

"It's a legitimate question to try to ascertain someone's health, especially knowing how grueling presidential campaigns can be," said Donna Brazile, a former Democratic National Committee chairwoman and a veteran of more than a half-dozen presidential campaigns. "It doesn't slow down if you win."

Beyond Sanders, Joe Biden is 76 and Elizabeth Warren is 70. If any of them won the nomination, the general election would feature two septuagenarians for the first time in U.S. history.

Trump began his term in 2017 as the oldest newly sworn-in president -- about eight months older than Ronald Reagan, who left the White House at 77 years, 349 days. Any of the three Democrats, including Warren, would set a new mark as the oldest new president. But Biden and Sanders have a potential additional distinction: Both would be older on their first day in office than Reagan was on his last.

Former President Jimmy Carter, who turned 95 Tuesday, has suggested there's a point at which it would be difficult for someone to manage the presidency. He recently said he hoped there was "an age limit" for the job.

"If I were just 80 years old ... I don't believe I could undertake the duties," Carter said, pointing specifically to responsibilities surrounding foreign affairs.

In the U.S., the current life expectancy is 76 for males, 81 for females -- though the expectancy is higher for married individuals and wealthier individuals, variables that apply to all the older 2020 candidates.

Some younger candidates, notably Pete Buttigieg, 37, have said it's time for a new generation to lead Democrats and the nation. Another hopeful, former Obama Housing Secretary Julian Castro, 45, took a swipe at Biden's age during last month's presidential debate.

In a sign that they're aware of the concerns, Biden, Sanders and Warren have pledged to release up-to-date medical records. They're also trying to project vitality.

While Sanders recovered from his heart procedure Thursday, his aides insisted he would participate in the fourth Democratic presidential debate later this month, a televised event that will require a dozen candidates to stand on stage for nearly 2 1/2 hours.

When asked about his age, Biden encourages voters to "watch me" and decide for themselves. He has said he'll get a physical before the Iowa caucuses, even jokingly asking a reporter recently, "You wanna wrestle?"

The former vice-president's aides bristle at questions about his fitness for office. They note he maintains a healthy diet, likes to jog in parades while campaigning and, like Trump, does not drink alcohol or smoke.

Warren has largely avoided age questions. She told reporters this week she tries to walk every day, sometimes while making phone calls or listening to audiobooks -- currently a Sean Duffy crime novel.

"I get out. I stretch out," she said. "My goal is 7 miles a day, but I don't always hit it."

The Massachusetts senator is the only one of the septuagenarian hopefuls who admits coloring her hair. Sanders and Biden are balding and white-topped. Trump insists his full crop is natural.

The president, who does not vigorously exercise and is a proud fan of fast food, raised eyebrows during the 2016 campaign when his personal physician released a glowing assessment stating "unequivocally" that if Trump were elected, he'd be "the healthiest individual ever elected to the presidency." Dr. Harold Bornstein later said Trump dictated the letter.

But Trump has family longevity on his side. His mother lived to age 88, his father to 93.

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Associated Press writers Will Weissert in Carson City, Nev., and Thomas Beaumont in Des Moines, Iowa, contributed to this report.