But election administration and legal experts said the timing of Biden's exit on Sunday makes it unlikely that any Republican ballot access challenges will succeed, with some calling the idea "ridiculous" and "frivolous." Democrats are on safe legal ground as they identify a new standard-bearer, they say, because the party hasn't officially chosen its nominee. That typically occurs with a vote of delegates at the party's convention.

"It's ridiculous for people to talk about 'replacing Biden.' He hasn't been nominated yet," said Richard Winger, a leading expert on state ballot access laws and the longtime editor of the "Ballot Access News" newsletter.

Talk of possible Republican legal challenges has been swirling since Democrats began discussing -- privately and publicly -- whether Biden should drop out of the race.

In June, a conservative Washington-based think tank laid out what it described as "the contentious path ahead" if Biden did not seek reelection.

"There is the potential for pre-election litigation in some states that would make the process difficult and perhaps unsuccessful," said Mike Howell, executive director of the Heritage Foundation's "Oversight Project" in a public memo. Howell identified three battleground states -- Georgia, Nevada and Wisconsin -- as possible places where pre-election litigation could be filed to try to block a presidential candidate from withdrawing.

Republican House Speaker Mike Johnson also raised the specter of legal challenges on Sunday morning, hours before Biden's announcement.

"I think they have got legal hurdles in some of these states, and it'll be litigated, I would expect, on the ground there, and they will have to sort through all that. They have got a real problem," Johnson, R-La., said on CNN's "State of the Union."

But several experts disagree. Trey Grayson, Kentucky's former two-term Republican Secretary of State, said there are no grounds to challenge the Democratic Party from picking a post-Biden nominee.

"It's a pretty frivolous claim," said Grayson, who also served as president of the National Association of Secretaries of State and chair of the Republican Secretaries of State Association. "The filing deadlines haven't passed yet, and the Democrats haven't nominated anybody yet."

Delegates from every state, territory and the District of Columbia select their parties' presidential and vice presidential nominees using a process outlined in national party rules. Even though Biden won enough delegates as of March 12 to unofficially clinch the nomination -- making him the presumptive nominee -- because he ended his campaign before delegates voted, the task of selecting the party's candidate for the November election still falls to convention delegates.

Had Biden stepped aside after the convention, a different process would have been triggered, one that could have been complicated by state ballot access deadlines.

"The parties control the process as to who their nominee is," said Edward B. Foley, a law professor who leads Ohio State University's election law program. "I just don't see how the Republican Party or anyone associated with the Republican Party would have any standing to bring any litigation in connection with this."

An Associated Press review of relevant state election laws showed that Biden's withdrawal Sunday came well before the presidential ballot access deadlines in every state that specifies one.

Of the three states mentioned in the Heritage Foundation memo, Wisconsin was the only one with a specific deadline: Sept. 3. Neither Georgia nor Nevada specifies a deadline for a candidate to be placed on the ballot.

The state with the earliest ballot-access deadline is Iowa, which requires parties to provide the names of their nominees no later than 81 days before the general election, or Aug. 16. The law further states that for late conventions, like next month's Democratic National Convention, which begins Aug. 19 and concludes Aug. 22, the parties have until five days after the convention's adjournment to report its nominees to the state.

A handful of states have their deadlines on Aug. 22.

Ohio had a deadline of Aug. 7, but the state eventually enacted a law changing the date to accommodate the Democratic convention, as it has done in previous years for both parties.

Before Biden dropped out, the Democratic National Committee had decided to move forward with a virtual roll call in early August, ahead of the convention. The DNC said the earlier roll call could head off any potential avenue for legal challenges because Ohio's law technically does not take effect until September.

In an interview with Rolling Stone released July 4, Heritage Foundation's Howell referenced statements Biden had made where he referred to himself as the Democratic nominee. "If he, in those statements, is legally arguing that he is the nominee, I think that should be read as him circumventing the formal convention process," he said in the interview. "And Biden's doing so has tremendous legal implications and statutory impact for states that specifically point to the DNC for who shows up on the ballot as the party's nominee."

Grayson disputed the premise that Biden's statement that he was the nominee carries any legal significance.

"Someone calling themselves the nominee doesn't make them the nominee," he said. "It's no different from Trump saying he's the nominee before the convention. He wasn't either."

Howell told the AP on Sunday, "We're deep in the preparation stages for our next move and won't be telegraphing it in the media."

"As you can understand, there is a massive dark money operation and some very conniving donors and power centers pulling strings right now," Howell said. "They read the news just like anyone else and they will find out about our next move when we make it."

Winger added that even if Biden had waited until after the nominee had dropped out, there is historical precedent for replacing a nominee on a presidential ticket. In 1912, Vice President James Sherman died six days before Election Day while running for reelection on the ticket with Republican President William Howard Taft.

According to news accounts, the Republican National Committee met after the election and selected Columbia University's Nicholas Murray Butler to replace Sherman, and Republican electors cast their votes for Butler instead of Sherman when the Electoral College met. The choice was not controversial, since Taft had already lost the election.

In 1972, Democratic vice presidential nominee Thomas Eagleton of Missouri withdrew from the ticket after the convention following his disclosure that he had undergone psychiatric treatment. The DNC met the following week and held a vote selecting Sargent Shriver, brother-in-law to the late President John F. Kennedy, to replace him.

"That was done in August, and nobody sued," said Winger. "It's just not a problem."

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Associated Press writer Christina Almeida Cassidy contributed to this report from Atlanta.