The House defended DOMA and OTL and failed, I doubt the justice department would do much better.
On the subject of Portman and Romney's views, I think it possible they could say something like "While Mitt Romney and Rob Portman differ on whether same-sex marriage is good, they both agree that the decision should be left up to individual states and legislative or democratic change, and not be forced nationwide by activist courts" or something like that. In fact, having Portman say that he is personally for marriage equality could be seen as a plus, making the "we just want states' rights/the people's will" seem less homophobic.
I just feel like that might be damaging to them, though. With one supporting and one not. I suppose there are ways they could spin it though.