Over the past three decades, both parties have had roughly equal electoral strength nationally, making control of Washington constantly up for grabs. Since 1992, the country has cycled through two swings of the pendulum, from united Democratic government to divided government to united Republican government and back again, with both sides seeking that elusive permanent majority, and attempting to sharpen the distinctions between the parties in order to win it. This also intensified partisanship.
These triple developments—the nationalization of politics, the geographical-cultural partisan split, and consistently close elections—have reinforced one another, pushing both parties into top-down leadership, enforcing party discipline, and destroying cross-partisan deal making. Voters now vote the party, not the candidate. Candidates depend on the party brand. Everything is team loyalty. The stakes are too high for it to be otherwise.
The consequence is that today, America has a genuine two-party system with no overlap, the development the Framers feared most. And it shows no signs of resolving. The two parties are fully sorted by geography and cultural values, and absent a major realignment, neither side has a chance of becoming the dominant party in the near future. But the elusive permanent majority promises so much power, neither side is willing to give up on it.