Polls are using incorrect methodology.

[Zynx]

They are consistently overweighting Republicans. According to the results of the 2000 exit polls 39% of voters were Democrats, 35% were Republicans, and 27% were independents. Those numbers don't add up to 100% due to rounding. Now, if we apply party breakdowns like we have seen consistently in polling we get something like this:

Democrats 39%- 90% for Kerry, 8% for Bush 2% for Nader
Republicans 35%- 91% for Bush, 7% for Kerry, 2% for others(including Nader)
Independents 27%- 47% for Kerry, 40% for Bush, 13% Undecided or Nader

That comes out to 50.24% for Kerry, 45.77% for Bush, and the rest are undecided. Basically both parties' partisans will vote 90% for their candidate and we have seen such numbers in polling so far, give or take a percent. Sometimes Kerry's lead among independents is larger than what I credited him for, but I think it averages out to about 7%.