If anyone out there's still undecided, please consider the effect of a Republican victory would have on each party's approach to governance.
Republicans have spent the past four years, starting with a meeting on Obama's Inauguration Day, uniformly opposing any Obama initiative, even if it's the Heritage Foundation's health care plan, a stimulus bill whose largest component is tax cuts, or John McCain's cap-and-trade system. By forcing the majority party to get to 60 votes all on its own to pass any major legislation, we guarantee a perpetually paralyzed government, only able to do anything when one party reaches a rare level of electoral dominance. Our system can only realistically work if legislators are willing and able to compromise. Rewarding Republicans for four years of obstruction would encourage the newly-minority Democrats to behave similarly against Romney.
Similarly, the Republican Party needs to be punished for its manipulation of voting laws. Voter ID laws disenfranchise real voters to combat supposed voter fraud. Republican legislatures in states Ohio and Florida restrict early voting, leading to long lines and more barriers to voting. They aren't able to show any real reason to do these things, leaving only partisan advantage as a plausible explanation.
A Republican victory means more of the same, as so often happens when some behavior earns a reward. But losing an election against an incumbent with a 7.9% unemployment would hopefully be enough of a wake-up call to cause them to re-examine their behavior. Our system just doesn't work if one party acts like the Republicans have.