The Romney "joke" about his birth certificate was no accident.  

It got birtherism back into the conversation about the election. It had dropped out, and had been replaced with the abortion discussion around Congressman Akin. No doubt Romney felt it was unfair to tag him with that (arguable, he did pick Ryan as his VP). So he hit back with birtherism. 

Democrats treat it as a moral issue. An appeal to Republican reasonableness, which does not exist. The best response is to not take the bait. Accept it as a joke. No more discussion. Change the subject.