Job Approval the Number to Watch
Presidential elections don't set up clearly until we're much later in the cycle, so the current polls really don't tell us much. Past research has shown that polls don't start to get it"right" on calling the race until September. But another indicator is useful, incumbent job approval.
There's a sort of rule of thumb in U.S. elections that when incumbent job approval drops below 50% s/he is in trouble. Maybe we've all overlooked this rule because Bush won such a close election finishing second in the popular vote count, so that fact that his job approval has been hovering around 50% for much of this year gets forgotten.
But the recent trend for him has been downward as this average of polls on the Real Clear Politics webpage shows. And sub 50% means that not just Dems but a fair number of independents now don't like his performance. If elections are retrospective evaluations of incumbents, which I think they are, this may be Bush's biggest source of concern today.