(via Armed and Dangerous, who got it from USA Today) A map of the counties where Bush and Kerry had majorities. Bush in red, Kerry in blue. This map seems to indicate Kerry support is biggest in urban and coastal areas, and to a lesser extent also to the banks of major rivers...
If the counties in Western states weren't so big, this map would probably show this clustering effect even clearer.
(If you'd draw a similar map of Belgium, I would probably be a red dot in a sea of blue)