Suburbs are a battle ground?

An interesting point from David Brooks in the NYTimes:

But the suburbs happen to be where this election is being fought — around Philadelphia, New York, Denver, Minneapolis and Columbus. The general rule is that Democrats win in the more densely populated suburbs close to the cities and the Republicans win the more sparsely populated ones farther out. The central fight in American politics now is over where the line is demarking the two zones, and the central Republican problem is that every time the party mobilizes its exurban base it further alienates the marginal voters in traditional suburbs where Congressional elections are won or lost.