I can’t believe that for all the times it’s come up, Romney has never offered any kind of alternative explanation as to who caused the economic crisis. Aren’t there plenty of sources he could point to to show how the bankrupt government entities Fannie and Freddie and the liberal policy from the Carter and Clinton era of giving mortgages to people who couldn’t pay them back caused it (Community Investment Act)? Policies that Obama himself fought for in his community organizing days when they harrassed banks to offer such loans.
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.