Some newspapers and other media are giving credence to a Heritage Foundation analysis that claims it would be difficult in some states for the Democratic Party to nominate anyone for President other Than Joe Biden. For example, see this New York Sun article. In reality, there would be no legal problem in any state. No state requires a qualified party to certify its nominees for national office earlier than August 21.
Furthermore, even if there were such a state law, it would be unconstitutional, under old U.S. Supreme Court precedents that say national conventions of major parties are not constrained by state election laws. In the 1972 Democratic national convention, the U.S. Supreme Court ruled that the Democratic convention had the authority to decide for itself which delegates to seat. There were two competing slates of delegates from Illinois. One set had been chosen in the Illinois Democratic primary; the other had not. But the U.S. Supreme Court upheld the authority of the Democratic convention to seat the unelected delegates. The party had national rules about diversity in the delegates, and the convention felt the primary winners from Illinois were not legitimate. Cousins v Wigoda, 419 U.S. 477 (1974).
In 1981, the U.S. Supreme Court ruled in Democratic Party of U.S. v La Follette, 450 US 107, that the national convention had the freedom of association right to refuse to seat delegates elected in the Wisconsin Democratic presidential primary, because Wisconsin used an open presidential primary and this contradicted national party rules. However, the national party, not withstanding its court victory, later gave an exception to Wisconsin and no longer objects to the Wisconsin open primary.
Furthermore, on March 4, 2024, the U.S. Supreme Court unanimously ruled in Trump v Anderson that the relationship between the people and the president is so vital to our form of government, it is intrinsically unconstitutional for a single state to keep a presidential candidate off its ballot, if that created a “patchwork” in which the candidate was on in some states but not others.
Presidential nominees get on the general election ballot with this paperwork: (1) a certificate from the chair and the secretary of a presidential convention, listing the nominees for President and Vice President: (2) a certificate from the state chair of each party listing the party’s presidential elector candidates and whom they are pledged to. None of this paperwork is forwarded to a state elections office until after the national convention is over.