Only 7 states still have strait party voting. Michigan and Nevada are probably the only 2 Biden would likely win, and could be effected by forcing voters to select individual candidates.
If RFK Jr can build enthusiasm, particularly in millennials, he could do to Biden what Perot did to HW Bush, though with a smaller share of the vote.
Down ballot loss of a few percent for democrats in 7 states is not a trivial impact.