Against Romney, Obama has a serious chance of losing, but losing isn't that bad. Against Gingrich, the chance of losing is much smaller, but defeat would result in a crazy person getting control of the presidency and with it the American nuclear arsenal. I'm not too worried that Gingrich would be able to pass major legislation to undo the social safety net, because I know how hard it is to get major economic proposals through Congress. He isn't an especially smart political tactician either, so I imagine a future of botched proposals that the left successfully smacks down. But it's harder to block bad foreign policy proposals than domestic proposals for some reason, so he might be able to create real havoc there.
The big thing that has come to interest me lately is the situation downticket. Romney most likely isn't going to turn into a chaotic mess that hurts Republicans everywhere. (You might think with him that Republicans wouldn't turn out to vote for a moderate. I don't think that's likely -- Republicans can be driven to hate Obama enough that they'll come out to vote against him.) But with Gingrich there's a serious chance that he'll just be unappealing enough to hurt the party as a whole. If Gingrich getting the nomination is worth one or two more Democratic Senators, that's enough to make me hope he gets it.