If there was a single issue that made a difference in the electoral count it was the gun issue. Bush dismisses it at his peril, particularly if he's running against someone who can claim a pro-gun record.
My point is that there
was no one single issue that "made the difference" in 2000 -- the election was so close that there were any number of such issues. And although we should assume the 2004 election will be close, it is extremely unlikely that it will be as close as the one in 2000.
Bush, to the best of my knowledge, has not changed his position on the gun issue from the 2000 campaign. The only thing that could conceivably hurt Bush, imho, would be if the AW Ban actually reaches his desk, and that seems unlikely; I expect it to remain bottled up in committee in the House until it passes its expiration date.