In the following sentence, should "Who", "Whom", "Whoever," or "Whomever" be used in the blank space?: He was not sure ___ he was voting for.

Whom is the best choice.

"Whom" is used for the object of a sentence. "Who" would be the subject of the sentence, or the one doing the action.

Since this is asking about the one being voted for, the action is being done unto them, making this the object of the sentence.