“Withholding a vote from Mitt is giving a vote to Obama.”
No.
Fantastical as is sounds, voting for Obama would be giving a vote to Obama. That is actually a very different thing than not voting for Romney.
Withholding a formerly solid Republican vote from Romney means the Republican base is depleted by that one vote, and Romney would be down by ONE.
Actually giving a formerly solid Republican vote to Obama means that Romney not only loses one vote but (it gets complicated here) Obama actually GAINS one vote, thus Romney would actually be down by TWO. He will have lost the vote that any other Republican could have relied on, but also Obama would have PICKED ONE UP.
I know word problems are really hard, but I tried to keep it simple for you.