As far as I know, there won't be a candidate for that party. If he waits a little longer, his name will be on all the ballots anyway, and we can have a new, improved 2000 fiasco, because votes for him will be thrown out.
The Party owns the ballot line in each state. The electors are the ones for whom you vote and they are pledged to the Party's candidate. Since the ballots haven't been printed in most states because the process of choosing candidtes for other offices is still going on, I don't foresee a problem with Kerry. The Dems replaced Eagleton with Shriver about this time in '72 with little difficulty.