Obama thanks you! as your vote will be another vote towards leaving him in office for another 4 years (by splitting the anti-Obama vote)
You do realize that you have a much, MUCH greater chance of dieing in a car crash on your way to the polls than you do of actually swinging the election, right?
Besides, Obama has become practically moot in the past few months and this has happened even though the Democrats control the White House and the Senate. If Republicans take back the Senate but not the White House, Obama will have no power. And at the very least, a split vote between the LP and the GOP will teach future GOP candidates to move toward the libertarian wing of the party.
Also, if a Mitt Romney or a Newt Gingrich is elected President, then the GOP is setting itself up to lose to another Obama in the next couple election cycles. Those are just the facts.
So by supporting anyone but Obama, you are in essence setting up this country for a continual cycle between terrible Presidents and bad Presidents.