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

A special counsel report investigating U.S. President Joe Biden's handling of classified documents spared the world leader of criminal charges on Thursday.

But the report still dealt a politically devastating blow to the incumbent just nine months away from the next vote: It made multiple, blunt notes about Biden's age and mental abilities.

In recounting five different interviews with the president, special counsel Robert Hur described Biden, then 80, as a "well-meaning, elderly man with a poor memory" — so much so, he wrote, that members of the public wouldn't convict him of knowingly committing a felony.

"It would be difficult to convince a jury that they should convict him by then a former president well into his eighties of a serious felony that requires a mental state of willfulness," Hur wrote in his lengthy report.

The comments set off yet another political firestorm around age and mental fitness in the U.S. presidential campaign, which, for the first time in history, will likely be 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, but doubt the public will cast their vote based on who they might prefer as a pinch-hitter in the event their first choice for president couldn't do the job.

"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 questions around his mental and physical fitness for the last several years, concerns fuelled in part by verbal blunders and public stumbles. Republican presidential candidate Nikki Haley has previously said a vote for Biden would be a vote for "President Kamala Harris."

Former president Donald Trump, who has faced a dizzying array of criminal indictments and questions around his mental fitness, continues to dominate the race to become Republican nominee against Haley, his only remaining major rival. In a recent speech, Trump, 77, confused his primary rival Nikki Haley with Democrat and former House Speaker Nancy Pelosi.

Polls have shown Americans are concerned about the mental and physical health of both men, but a new survey for NBC News said they 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.