It used to be easy to explain the relationship between the voting constituency of each party and the positions its politicians took in policy debates: Democrats are the party of the poor and favor big, redistributive government, while Republicans are the party of the rich and favor small, business-friendly government. But even though economic class is no longer a reliable guide to how Americans vote, party leaders remain committed to very different policy goals and visions—foreshadowing a bitter debate over the federal budget this year, as I explain today in Bloomberg Opinion. (The piece is also available via the Washington Post.)