Interestingly, viewers also thought Gore won the first Bush-Gore debate in 2000 -- when they were asked about it immediately afterward. When viewers were polled a couple of weeks later, Bush was identified as the clear "winner."
I don't know what kind of dynamic is at work that ends in that kind of result, but someone suggested on Don Imus' radio show earlier this week that this is often the case.
A conservative frienf who sat in the gallery for the Oliver North hearings in 1987 commented on this issue. He said that when you read the transcripts of the hearings, you walked away somewhat appalled at how dysfunctional the White House had gotten. When you heard North testifying, you were ready to shout "Amen!"
Or, as he summed it up: "True North is not Magnetic North."