I think Romney knows that's a losing game in a debate. You can't go back and re-try the experiment with everything the same but one variable changed to prove the cause, so people will hear what they want to hear. It's best to let it go and focus on what you can easily defend.
At the very least he should say it was a bipartisan problem, and that both parties did not agree to reform Fannie and Freddie’s mortgage lending practices when they needed to. He just lets Obama completely tar the Bush administration for the financial crisis, and uses that as his basis for saying we can’t go back to “Republican” rule. To me it’s a losing proposition to NOT challenge Obama on that.