2000 Bush 48.0 47.9 +0.1 Gore 46.0 48.4 -2.4 Nader 4.0 2.7 +1.3
Gallup has Bush up 51 to 46. That tells me that Bush wins by 53 to 45.
So if you apply the 2000 numbers to 2004 (Bush for Bush, Kerry for Gore), the result is Bush 50.9, Kerry 48.4.
You're right. I think each candidate will get a little less than that, I'm predicting Bush 50.1, Kerry 48.1, others 1.8.