In my Introduction to The Almanac of American Politics 2010, I noted that Barack Obama was elected by a top-and-bottom coalition: he carried voters with incomes below $50,000 and above $200,000 and lost among those in between.
Tom Edsall, longtime political reporter for the Washington Post, has a fine piece in the New Republic blog on this phenomenon. He notes that low-income voters tend to like Democrats’ health insurance proposals but dislike their cap-and-trade proposals, while upper-income voters tend to take the opposite stand on each. It stands to reason that it’s harder to hold together a top-and-bottom coalition than it is one that is topheavy in the middle of the income spectrum, as John McCain’s was in 2008. But of course Obama’s coalition was larger.
This is not something new. Over their long history, the Republicans have tended to assemble more homogeneous and the Democrats have tended to have more heterogeneous coalition. As a result, Democratic coalition are often larger, but harder to hold together than Republican coalitions.