Bush's polling numbers aren't looking too good these days. Wesley Clark is beating him (albeit, within the margin of error) in a head-to-head poll, with all other major Democratic candidates managing statistical ties within the margin of error against Bush.

If you look at Bush's reelect numbers, they're generally pretty dismal as well. No polls show him besting 50% against a generic Democrat, and many show him below 50% against Democrats with far, far lower name recognition. Whenever an incumbent is below 50%, that's a warning sign. It's doubly a warning sign if the incumbent can't even garner a plurality.