Voters in the biggest cities are almost exactly balanced out by rural areas in terms of population and partisan composition.

16%  of the U.S. population lives outside the nation's Metropolitan Statistical Areas.  Rural America has voted 60% Republican. None of the 10 most rural states matter now.

16%  of the U.S. population lives in the top 100 cities. They voted 63% Democratic in 2004. 
The population of the top 50 cities (going as far down as Arlington, TX) is only 15% of the population of the United States. 
 	
The rest of the U.S., in Suburbs divide almost exactly equally between Republicans and Democrats.