With aging U.S. presidential hopefuls, how much do running mates matter to voters?

Over the course of Thursday's televised debate, incumbent U.S. President Joe Biden and former president Donald Trump disagreed, unsurprisingly, on virtually every key issue of the campaign fromabortion, inflation andimmigration toforeign affairs toclimate change.

But theissue looming largest over the night was the candidates' age.

A halting performance from Biden, 81,had some panicking Democrats questioning whether he should be replaced on the ballot. Others said it was just arough start due, in part, to the president suffering a cold getting an undue amount of attention in the press.

The issue has far overshadowed the performance by Trump, 78, despite the candidate havingmade a number of baseless claims throughout the event.

The debate solidified age and mental fitness as key concerns for voters in the U.S. presidential campaign, which, for the first time in history, is being led by two men who would be in their 80s by the time their term ends.

Political scientists say the aging slate might see voters pay more attention to vice-presidential candidates than they usually would, though they doubt the public will cast their vote based on who they might want to step upin the event their first choice for president couldn't do the job.

WATCH | Biden tries to shake concerns about age after debate:

Biden tries to shake off debate disaster with fiery campaign speech

2 months ago

Duration 3:25

A day after his disastrous debate performance prompted calls for him to step aside, Joe Biden delivered a fiery campaign speech in North Carolina aggressively attacking Donald Trump while acknowledging his own age and shortcomings.

"It's not really realistic to expect an American voter to say,'I'm really concerned about Biden's mental acuity, but it's OK. I'm going to vote for him knowing that, if he has to step down, [Kamala] Harris is there,'" said Renan Levine, who teaches political science at the University of Toronto's Scarborough campus.

"I am very skeptical that, even with more attention [on age,] that the vice-president will loom large in many voters' minds."

Biden has been plagued by persistent questions around his mental and physical fitness for the last several years, concerns fuelled in part by verbal blunders and public stumbles.

Trump, a convicted felon who has faced a dizzying array of criminal indictments, has also faced questions around his mental acuity. He previously confused his primary rival Nikki Haley with Democrat and former House speaker Nancy Pelosi.

Asked about his age during Thursday's debate, he said he had "aced cognitive tests" and then bragged about his golf scores.

Polls have shown Americans are concerned about the mental and physical health of both men, but asurvey for NBC News in February saidthey were particularly worried about Biden: Three-quarters of respondents, including half of Democrats, expressed their concern with his overall fitness.

If a president were ever unable to perform their duties as president, it would be their vice-president's constitutional duty to step into the role. In a race where both candidates are as old as they are, Levine said, it would be reasonable to think a vice-president might have a higher likelihood of being called up.

But the majority of U.S. voters typically don't decide their vote based on the vice-presidential candidate, even in a race where they might become more.

"Most Americans, we have to remember, especially in this moment, vote for the candidate of the party that they usually or habitually vote for," said Levine.

"It's not like people go, 'Oh, well, who's the president? Eh, don't love him. Then who's the vice-president?'"

Democrats have pointed to Harris, 59, as the next logical successor to Biden. Chad Griffin, a member of the campaign's national finance committee, said the White House was fortunate to have a vice-president that is "tough as nails" though she's a frequent target for Republicans, who say she's waiting in the wings in caseBidenisn't able to complete a second term.

"Vote JoeBidentoday, get Kamala Harris tomorrow," read oneadvertisementfrom Trump's campaign.

Trump has said he's picked a running mate, though he hasn't announced who.

WATCH | Harris decries special counsel report:

Vice-President Harris calls special counsel's report 'gratuitous, inaccurate and inappropriate'

7 months ago

Duration 1:49

Coming to U.S. President Joe Biden's defence, Vice-President Kamala Harris decried special counsel Robert Hur's report questioning Biden's mental acuity, calling it politically motivated and saying they expected Hur to have acted with 'a higher level of integrity than what we saw.'

Paul Quirk, a professor of political science at the University of British Columbia, said the question becomes whether the presidential candidates' age is a big enough issue to push voters to pay more attention to the second-in-command.

"Does the vice-presidential candidate matter when he or she is much more likely than usual to become president? This is a new circumstance," said Quirk.

Vice-presidents walking fine line

In recent campaigns, vice-presidents are typically used to rally the party's voter base and shore up support for the presidential candidate. Harris and the Republican nominee's running matewill each have to walk a fine line of standing behind their respective potential commander-in-chiefwhile making plain theirown ability to serve.

"That's going to be the delicate balance that they're going to need to demonstrate throughout the rest of this year," said political science professor Beth Fischer, referring to the Biden-Harris strategy.

That's a message ambitious candidates already know how to deliver, Quirk noted.

"They would be emphatically expressing confidence that the presidential candidate will serve out the term while declaring that they are prepared if, somehow, he doesn't," he said.

"But that's what they always do."