At the core, a political party has the right to determine how they choose their candidate. The justices know a ruling that would keep her off a ballot, or some ballots, would spark massive civil unrest. While true that some of the justices have demonstrated a distain for actual legal rulings they don't like, I don't see this being even a remote possibility.
https://www.factcheck.org/2024/07/experts-delegates-free-to-pick-democratic-nominee/
Moreover, political parties have the constitutional right to determine the procedure by which they select their nominees, as repeatedly confirmed by the Supreme Court, Foley said, citing the Supreme Court cases Democratic Party of United States v. Wisconsin ex rel. La Follette and California Democratic Party v. Jones.
The authority of the national parties to choose their nominee in the event the nominee cant run comes as a surprise to many in this day of wall-to-wall primaries, Elaine Kamarck, author of Primary Politics: Everything You Need to Know about How America Nominates Its Presidential Candidates, wrote in September. And yet, it is a reminder that the choice of a nominee is party business not state law, not federal law, and not constitutional law.
Foley said there would be no basis whatsoever for Republicans (or anyone else) to challenge the Democratic Partys decision to follow its own rules in nominating someone other than Biden.
According to the Democratic Partys rules, Foley said, the presidential preference primaries determined who the convention delegates are; the primary voters did not directly choose the partys nominee. Bidens now having voluntarily withdrawn from the race before the delegates nominated him based on his status as presumptive nominee as a result of the primaries, the delegates are free pursuant to the partys own rules to choose a different person as their nominee. There has been no disenfranchisement of primary voters as a part of the process of the party following its own nomination rules.