Chait anticipates that the election won't be that nasty after all:
The biggest constraint on a campaign's ability to drive a message is the general sense of what is in or out of bounds. And that sense, in turn, is largely defined by the willingness of members of a party to call out their own candidate for going too far. The socially liberal, economically conservative sensibilities of the party elites are working in tandem to hold back Republicans from attacking Obama on cultural grounds, and to at least complicate Obama's populist attacks on Romney's business career.
Scheiber notes that such restraint is probably calculated:
As on the right, I think you see Democrats making a pretty sober calculation about which message has the best chance of succeeding. And the problem with portraying Romney as unprincipled and cynical is that it leaves open the possibility that he might really be a moderate, or at least govern as a moderate, which would be reasonably attractive to swing voters. Much better to argue, as Obama has, that there’s no reason to doubt that Romney believed what he said when he took tough stances on immigration and spending and social issues during the GOP primaries. He’s a man of conviction after all—just the wrong kind.
All of which is to say that, while elites may have some leavening effect on the campaign discourse, the relative civility is really a product of each side calculating that the nastiest messages simply aren’t the most effective, if for relatively idiosyncratic reasons.