Right now, Obama needs to start putting out a bunch of positive ads, listing his accomplishments, like Clinton in 1996. If Romney and Gingrich go after each other, it works to Obama's advantage as he can focus on the general election. It's also very important for him to get his approval rating back up near 50.
Ads aren't going to do anything for Obama's approval rating. It may translate into a temporary bumb, but nothing that will last for months or until election day.
Obama would be better served saving his money until the campaign, or else attacking Romney and trying to get the primary to drag out longer.
