It is not a flip-flop when circumstances significantly change. There was no need for such an Amendment prior to a two months ago, but then activists judges and renegade mayors got into the picture...
Obviously, the "Flip-flop" label is hurting Kerry, so he has to try to reverse it on Bush. Just like everything was Bush AWOL on Job Creation, Bush AWOL on catching Ben Laden, Bush AWOL etc.
Since that's not working anymore, now they have to find a new counter-attack. I think that POTUS was clear that he was leaning towards a Constitutional Amendment, but that he was hoping that the State's might handle this correctly. Now it seems that we can't count on that, so POTUS is calling for Federal action.