Thank you very much for the various answers. I don't think such a scenario is actually plausible, but I was curious as to the conditions and legalities of such an occurrence.
I appreciate your help! :)
Perfectly legal. Voters are actually voting for already know party-loyal electors. The candidates' names are on the ballot with different deadlines, only as a matter of convenience for the voter, and advantage for the party.
As a practical matter, the election has to be close enough to win with some risk of voter confusion (but then, widow Carnahan won, albeit same last name), but lots of DEMs will pull the DEM lever, no matter whose name is on there. Check some ballots, you will see that party affiliation is marked prominently.