Whom. Whom will you vote for in November?
"for" is a preposition, and "for whom (will you vote)" is the understood prepositional phrase, so we should choose the objective form of the personal pronoun. This is sixth-grade stiff, for crying out loud!
When even journalists, our university-trained word merchants, can't seem to get their heads out of their asses long enough to put together a proper interrogative statement, we are reminded of the reasons that world policy depends upon successful marketing of the sensationalistic whims of the cradled, consumerist, disinterested citizens of a pseudodemocracy who, given a choice among 320 million individuals, manage to find two of the most narcissistic, greedy, and spiteful among them to represent themselves to the world.
Sick, sad world. We chose them; we deserve them.
(Or, perhaps I should say, "us chose them." Probably no one would notice the difference.)