Why do you say that? It's actually based on an educated supposition. Most people who are hard-core evolutionists (i.e. they don't even believe in "medial" positions like ID or theistic evolution) tend to be very secular in their worldview. Most secularists tend to vote Democratic, since most secularists are either liberal in general or else are "scared of the Religious Right." Ergo, it follows that most people who are evolutionists vote Democrat.
I am reminded of the story Mark Lowry tells about evolutionism vs creation. Evolution and the Big Bang theorists tell us that some six billion years ago, there was NOTHING. Then out of NOTHING, this big bang happens and suddenly there’s Chaos. Then, magically there comes ORDER from Chaos.
So he offered to take his watch apart, put the components into a paper sack and shake it for the next six billion years. Then he’d take bets (he’s a Baptist, after all) about whether or not he could reach into that bag and pull out a watch that was somehow all back together and keeping good time or a handful of parts.
I think that explains the situation quite clearly.