I think they broke half and half.
I pretty sure in 2000, 1992 they broke for Gore and Clinton
in 1976 and 1980 the broke for Ford and Reagan, except there weren’t enough for Ford.
HEre is the 2004 exit national poll data...scroll down most of the way and it shows how it broke when they decided:
http://www.cnn.com/ELECTION/2004/pages/results/states/US/P/00/epolls.0.html
Kerry had a slight advantage from those that decided in the last 3 days, but Bush had a tiny advantage in the last week.
This shows 9% decided in the last week (when last week and 3 days out and day of voting all combined)...
Actually, I think the undecided are decided and just dont want to say who they are voting for. I work the phones for McCain and spoke with an elderly woman yesterday who sounded absolutely terrified to divulge who she was voting for. Probably all that in “your face” talk. Also, the pumas are telling the pollsters they are voting bho. Who knows what the numbers really are.