To make me vote for Obama.
I have zero desire to vote for Obama. I think he has been a disaster from a progressive standpoint on just about any issue (women's rights and maybe gay rights withstanding, but even for those he's really just not horrible at his best). I had no plans to vote the top of the ticket at all. But it seems like every week the GOP in general and the Romney campaign specifically do something that makes me consider acceptable voting for a president who assassinated an American citizen.
The latest, of course, is the horrid choice of vice president.