"Study Who(m) & What to Vote For"

       Does it make any difference?  With or without the "m"?  This is a note I jotted myself before the recent election.

       No, I was going to be doing the same thing either way.  But that darn "who, whom" question still tickles the corner of my mind.  And I didn't feel right till I added the "m" as object of the preposition "for."