What would happen if Biden or Trump drop out of presidential race? Who would run?

Election Day 2024 is expected to be headlined by a rare presidential rematch between the oldest candidates in the nation's history: 81-year-old President Joe Biden and 78-year-old former President Donald Trump.

Even before the July 13 assassination attempt on Trump, there had been talk about the duo's ability to serve based on age, cognitive ability, and in Trump's case, a potential prison sentence. All invite questions about what would happen if either nominee needs to be replaced by necessity or choice.

This brings into play the fragility of  American democracy, said Saladin Ambar, professor of political science and senior scholar at the Eagleton Center on the American Governor.

Story continues below photo gallery.

"We are just one outlandish scenario away from creating a whole matrix of problems," Ambar said. "These are things we should be talking about not because we want to be morose, but because there is so much on the line."

Who would run for president if something happened to Biden or Trump?

The White House announced that Biden tested positive for COVID-19 on Wednesday, adding that he is "experiencing mild symptoms."

Though firmly the presumptive nominee, Biden has not yet been officially designated as the Democratic nominee for the 2024 presidential election. The Democratic convention is set for August in Chicago. However, there have recently been reports that party officials are seeking to accelerate or possibly delay the nomination process, depending on their support for Biden.

On the other hand, GOP delegates have already formally nominated Trump for president. They did so on July 15 at the Republican National Convention in Milwaukee.

Nov. 5 would be the third consecutive ticket-topping election for Trump, who would join a small group including Henry Clay and William Jennings Bryan as three-time major party presidential nominees to make it to Election Day.

He almost didn't.

Just two days before securing the nomination, Trump joined more than a dozen presidents and presidential candidates who have been targeted by gun-wielding attackers. The first was former President Andrew Jackson, who survived unscathed on Jan. 30, 1835.

If Biden were to resign, die or otherwise lose his ability to serve, the 25th Amendment says Vice President Kamala Harris would assume the presidency. However, the rules for nominees and presumptive nominees are more complex and must abide by election laws and political party protocol.

More: What went wrong? Experts question Secret Service protocol for Trump rally

Since Trump has already been nominated, his replacement scenario is slightly more clear. The Republican National Committee would essentially have two options: select the candidate or hold another national convention, according to party rules.

Trump's vice-presidential pick, J.D. Vance, would be a clear contender, Ambar said. However, he would not be an automatic pick and there could be a series of potential candidates making their case to take Trump's or Vance's place on the ballot. People such as Tim Scott, Vivek Ramaswamy or Nikki Haley would be in the mix. The odds of a second convention would be slim to none, according to Ambar.

In Biden's case, Democratic delegates elected during Biden-dominated primary elections, conventions and caucuses and pledged to him would still be tasked with formally nominating a candidate at their national convention if he was no longer able to run. It would come down to the politics of the moment, how party leaders feel and possibly what Biden himself has to say, Ambar said.

"In the 21st century, politics are candidate centric. When the candidate is no more, we resort to an earlier process in American politics that is party centric and boss centric," he said. "It becomes about who the leaders are in the party."

It would be a similar process to one Republicans would have faced if Trump had been assassinated.

A replacement nominee would likely be chosen through a contentious process. Public speeches, negotiations on the convention floor and meetings behind closed doors could feature in a throwback process that many consider undemocratic in the modern system that relies on caucuses and primary elections to circumvent party bosses, Ambar said.

The process for the Democrats is also complicated by the existence of "superdelegates," unpledged delegates who could sway a contested convention, with people such as Harris, California Gov. Gavin Newsom or Michigan Gov. Gretchen Whitmer all lobbying for support.

"In the literal sense of the word, it would be a free-for-all," Ambar said.

What happens if Biden drops out of the race?

In 1968, Senator Robert F. Kennedy was assassinated as he was battling Vice President Hubert Humphrey and U.S. Senator Eugene McCarthy in the race for the Democratic nomination following Lyndon B. Johnson's withdrawal that March.

Kennedy was shot three times on June 5, just hours after winning the California primary, His 393 total delegates became uncommitted. U.S. Senator George McGovern offered himself as a replacement for Kennedy and a successor for his delegates two weeks before the convention. Yet, the delegates split themselves among the candidates. The largest group backed Humphrey, who won the nomination despite not entering a single state primary.

In 1972, Senator Thomas Eagleton was the Democratic candidate for vice president when he withdrew from the ticket after the convention. Eagleton, who reacted in response to reports that he had been treated for mental illness, was replaced by former U.S. Ambassador to France Sargent Shriver during a special meeting of the Democratic National Committee.

A similar scenario is in play if Biden drops out of the race after gaining the nomination at the Democratic National Convention. Party protocol calls for the national committee to consult with Democratic leaders, including members of Congress and state governors, before finding consensus on a replacement nominee or an entirely new ticket.

More: 'He never gives up': NJ woman witnesses attempt on Trump's life

For both Biden and Trump, changing the names on voters' ballots could be impossible depending on the timing of events. A move to change the ballot to reflect the candidates would be inevitable to ensure every voter was informed, Ambar said. However, regardless of the names on the ticket, voters in presidential elections vote for party electors in the electoral college and ballots denote the party. Congress could delay the election with the president's approval.

However, Ambar said that it would likely be tested in the courts. Moreover, the president's end of term is firm. The 20th Amendment stipulates the president and vice president must vacate their positions at noon on Jan. 20. Congress typically convenes on Jan. 6 to count the electoral votes and certify the election results.

In 1872, Democratic presidential candidate Horace Greeley died after the election, but he had already lost to Ulysses S. Grant and the electoral votes had not yet been cast. The party electors were free to vote for another candidate or Greeley. Congress chose not to count the votes for Greeley. It is unclear what would have happened had Greeley won. Electors might have taken the initiative to cast votes for their preferred replacement, or they might have united around a replacement designated by party leaders.

This article originally appeared on NorthJersey.com: What happens if one of the presidential nominees drops out of the race