Is it who or whom should I vote for?

You should use "who" when referring to the subject of a sentence and "whom" when referring to the object. In the phrase "who should I vote for," "who" is correct because it is the subject of the implied action (the one doing the voting). However, if you were to rephrase it as "For whom should I vote?" then "whom" would be appropriate as it refers to the object of the action.