There's no doubt that at the moment, Mitt Romney's greatest strength is the idea that as a successful businessman, he will do a good job stewarding the American economy. In fact, that may be his only strength.
Kevin Drum doubts "Romney's business schtick is really such a good one for him":
After all, when was the last time America elected a president whose background was primarily in business? That would be — never. I mean, sure, Bush Jr. rounded up investors for a baseball team and Jimmy Carter was a peanut farmer, experiences that they used as part of their resumes, but they basically ran as politicians. The last person to seriously run as a businessman was Ross Perot, and that didn't work out so well.
Tomasky suggests attacking Romney for his work at Bain:
[T]he argument is not “Mitt Romney was a job destroyer.” Because sometimes, Mitt Romney was a job creator. That’s an unwinnable argument. The winnable argument is that Mitt Romney worked for the 1 percent. Sometimes it helped workers, but other times it hurt them; but nearly every time, the wealthy investors, and Romney himself, came out all right. That’s the story to tell. And any story that paints a darker picture of Bain than is fair will be pounced on and ripped to shreds.