Oh, it’s on now. Mitt Romney and Newt Gingrich are going hard at each other in what may be becoming a really nasty GOP race.

They’d better be careful: One of the very plausible outcomes of a food fight between the top two contenders in a multicandidate race in which there’s little to separate the candidates is that voters will be disgusted with both and find a new horse. Could Rick Perry — or even Rick Santorum — wind up benefitting?

Here’s the latest. Romney today demanded that Newt Gingrich return the $1.6M he earned from Freddie Mac. In return, Gingrich sniped back:

If Governor Romney would like to give back all the money he’s earned from bankrupting companies and laying off employees over all his years at Bain then I would be glad to then listen to him. And I will bet you $10, not $10,000, that he won’t take the offer.

After repeatedly pledging to stay positive, Gingrich is now signaling that he is willing to push back hard on Romney’s attacks, even citing Romney’s days at Bain Capital — which are central to the Democratic case against Romney — to damage him.

Watch out! Even with only three weeks remaining until the caucuses, it’s not too late for Iowans — who have a strong reputation for disliking negative campaigning — to punish both of them. That may have been what happened on the Democratic side in 2004, when Dick Gephardt ran hard against Howard Dean, successfully scuttling both of them.

The question then is who would benefit from this effect, if it happens. That’s a tricky one. Ron Paul is running negative ads against Gingrich; Michele Bachmann has a new line about “Newt Romney” that she’s repeating constantly. So presumably neither of them is a logical landing point for those whose sensibilities are offended by attacks.

That leaves Rick Santorum and Rick Perry. Neither is exactly pure, but they’re probably the two who are best positioned to take advantage of such a dynamic. And both have other things going for them: Santorum has put in lots of time in Iowa, while Perry is inundating the state with TV ads. If one of them does surge, he could easily wind up in third place — or even better. A brand new ARG poll over the weekend has Perry only nine points out of first place in Iowa. I’m not predicting anything, but if I had to bet on a late surge from someone in Iowa, it would be one of the Ricks.