A lot of things can happen between now and November. Kerry may have a lead in the polls at the present time, but he hasn't been through the election meat-grinder yet.
Most Americans prefer a moderate candidate for President. So far, Kerry hasn't faced any really tough questions about his voting record in Congress, which was the most liberal of any member of that august legislative body. The economy is improving and it appears the transition of power to the new Iraqi government will proceed according to schedule. Indeed, I think Kerry will face more of an uphill battle to win or retain votes as the election enters its final two months than Bush will.