It's a question of intelligence and education. Understanding and accepting the theory of evolution, or rather, Modern Synthesis, as it is called today is a pretty basic scientific fact. If we've somehow made it through school but still have failed to grasp this pretty fundamental tool in understanding modern science, and in extension our world, then maybe we're just not cut out to axle the roll of running a country, let alone the richest country on earth.
To put it a little bit more succinctly. Being Creationist today, is however incredible a pretty strong evidence that the candidates elevator doesn't go to the top floor, or that he or she doesn't have all the necessary Indians in the canoe for the mission at hand.
Sure, candidates will pick any religion they think will get them the more votes. But if a candidates goes for Creationism. I'd feel insulted that they'd think that that could buy them votes in 2008. And if they can, we're in a really sad situation.