Ace, of Ace of Spade HQ blog, calls it the "preference cascade."
Once you decide to vote for a candidate because you agree with them on Issue A you accept their positions on B, C, D etc. and so your partisanship solidifies behind them.
I agree with that theory and build upon it.
People were looking for a reason to vote against Obama but we were told to vote against Obama would be bad. Either the rich will get richer or women will die because birth control costs $18,000 or you are a racist.
All the while the polls held Obama ahead by 7 points. I think--at best--Obama was ahead by 1 during this time with a 10-point pool of undecided Romney leaners.
After the debate people Romney gave people a reason to accept his Issue A and with it B, C, D, etc. and Obama provided plenty of reasons to reject his re-election from 4 years of failed policy to shattering the myth on live TV that he is a brilliant orator and smooth-mover.
I'm convinced Romney now leads with a minimum of 53 with less than 5 points worth of undecides. Those last few undecides are watching to make sure Romney's debate performance was not a one-off event or they will make the final buy-in once the polls consistently show Romney above 51% because then they can hide their Obama rejection amid the surge of support.