Just as Romney emerged victorious in the Republican primaries thanks to no one candidate monopolizing the anti-Romney vote, Obama might win a second term thanks to splitting of the anti-Obama vote. Romney's campaign has been heavily focused on trashing Obama's record, selling himself as a way to fight back against Obama. But if there is more than one major candidate not named Obama, like Gary Johnson, Romney's approach fails.
It's an unfortunate effect of our two-party system that all a candidate has to do to win is make his opponent untenable. This encourages negative attacks (like those Democrats unleashed on Romney today) instead of constructive debate.