After Robert Hur's Biden age comments, could any Democrats really take his place in 2024?

Thanks largely to a special counsel report, longstanding anxieties about President Joe Biden’s vigor have reemerged as a campaign issue at a moment when Democrats would rather talk about anything else.

The 81-year-old Biden bristles at suggestions that he has lost a step. Meanwhile, his White House staff, reelection campaign and other Democratic officials rush to his defense.

“I talk to President Biden regularly. Usually several times in a week. His mental acuity is great, it’s fine, it’s as good as it’s been over the years," Senate Majority Leader Chuck Schumer, D-N.Y., told reporters Tuesday morning. "All this right-wing propaganda that his mental acuity has declined is wrong."

Republicans are making as much noise as possible about Biden's fitness and have called for the 25th Amendment to be invoked or for him to leave office.

But voters also continue spell out that the president's age is a significant concern as he faces a likely rematch against former President Donald Trump.

A new ABC News/Ipsos poll after the report's release last week found a startling 86% of Americans believe Biden is too old to serve another term.

The same survey found 59% believe both Biden and Trump, who is 77, are too old for a second term.

No prominent Democrats have demanded that Biden bow out, but as speculation mounts about his ability to serve, there is bound to be interest in who among them could take the party's helm.

Here's a rundown of the who's who.

Joe Biden

That’s right: There is really only one person who can defeat Joe Biden in the primary for the nomination ahead of the Democratic National Convention this summer, and that’s the president himself.

Any last-minute candidate jumping in the 2024 contest now has an almost impossible pathway to victory. That's because the filing deadlines for primary ballot access will be closed for all but six states by the end of February.

And if someone were to enter the contest and get on all the ballots in all of those states, Biden’s delegate lead will likely grow on March 5, Super Tuesday.

The most likely situation in this hypothetical is Biden announcing he won’t seek a second term, and as king or queenmaker releasing his delegates for what would be a raucous horse race to the convention in Chicago.

So who are among the possible contenders?

Kamala Harris

If Biden were to step aside or physically be unable to be the nominee, the natural successor would be his vice president, Kamala Harris.

The 59-year-old California Democrat was Biden's first major choice as party leader in 2020, and she would be the easiest fill-in given that the campaign is often billed as the "Biden-Harris" ticket.

As USA TODAY White House correspondent Francesca Chambers pointed out in a story last year, Harris began stepping into the role of progressive culture warrior and has been at the forefront of other major issues after initially struggling to find footing in the vice president's role.

There are misgivings about Harris's ability as a campaigner given how she dropped out of the 2020 primary before the Iowa caucuses and her anemic poll numbers.

But any overt attempt to dismiss her as the rightful heir to the nomination in this scenario would infuriate her supporters and possibly alienate a significant part of the Democratic base: Black voters.

Dean Phillips

If there's anyone waiting to say 'I told you so' in 2024, it's Rep. Dean Phillips, D-Minn., who officially challenged the president in the primary.

The 54-year-old congressman's campaign has been about America turning the generational page, which he eagerly points out in the context of this recent drama.

"I saw the forthcoming disaster early on, spent a full year encouraging a competition, and only entered the ring when no one else would," Phillips said in a Feb. 11 post on X, formerly Twitter.

"And while the party people hate me for trying to protect us, I love you all nonetheless."

But Phillips was trounced in the South Carolina primary, coming in third with less than 2% behind author Marianne Williamson, who recently dropped out. He has been bombarded with questions about why he even remains in the race, and it is hard to imagine how he could be seen as a viable general election candidate by Biden's delegates.

Blue state governor's gang: Gavin Newsom, J.B. Pritzker, Gretchen Whitmer, Andy Beshear

One crop of candidates where Democrats could find an alternative to Biden is at the state level. A number of blue-state governors have been testing their national brand.

And they've been putting money in their coffers too; courting national donors, opening PACs and creating the infrastructure for a possible presidential bid.

Chief among them is California Gov. Gavin Newsom, who has been accused of running a shadow campaign for 2028 amid his robust defense of Biden.

Other governors that progressives would kick the tires on include Illinois' J.B. Pritzker, who has the money and strong surrogates who could muscle a convention run.

Another would be Michigan Gov. Gretchen Whitmer, who was once on the VP shortlist and has been a loyal Biden supporter amid the backlash over the president's handling of the Israel-Hamas war that has devastated Gaza and angered Arab American voters in her state.

Whitmer has racked up significant legislative wins with a newly minted Democratic legislative majority, and she represents a swing state to boot.

But there are also dark horses who might get a push from more moderate corners of the party, such as Kentucky Gov. Andy Beshear, who caught national Democrats' eye with his victory in the conservative-leaning state.

Beshear, who has been mentioned as a potential vice presidential pick as well, also recently launched a federal PAC aimed at races across the country.

What about Pete Buttigieg, Michelle Obama and the 'superdelegates?'

Outside the usual suspects who appear to be jockeying for at least a future presidential run, there would certainly be an attempt by other factions of the party to elevate their preferred candidate should Biden bow out.

Polls would be shopped around for weeks, including candidates who ran last time and have maintained a decent following among the liberal base, such as Transportation Secretary Pete Buttigieg, who won the Iowa caucuses in 2020, or Massachusetts Sen. Elizabeth Warren.

One could also fathom a campaign to recruit candidates outside Washington, such as former first lady Michelle Obama, who has expressed zero interest in running, or Dwayne "The Rock" Johnson, who said he has been approached about running.

In all, 4,532 delegates will be assembling at the Democratic convention, including 744 “automatic” delegates, famously known as “superdelegates” who will get a vote should there be a second nominee ballot.

Those host of party leaders − former and current − plus elected officials will want to have a say as well.

And that doesn’t include behind-the-scenes maneuvering and input from grassroots organizations and major donors.

Whatever unfolds about Biden’s age in the coming weeks and months, any attempt at crowning a new Democratic nominee would be a messy affair.

This article originally appeared on USA TODAY: After Special Counsel report, who could replace Biden on 2024 ticket?