Using economic growth (as measured with per capita real disposable income growth between the 2nd quarter prior to the election year and the 2nd quarter of the election year), presidential Gallup approval on Labor Day (I'm guessing Bush will be at around 30), a variable measuring how many terms the president's party has held the White House, and a variable measuring whether there is currently a war going on, I predict...
John McCain will receive 47.7% of the two-party vote.
This estimation has a standard error of 2.4, giving McCain roughly a 16% chance of getting over 50% of the vote.
Keep in mind all that this equation doesn't account for (campaign quality, the color of the candidates' skin, media coverage, etc.). Still, I nailed the 2004 prediction.