For much of American political history, thus, the critique of the two-party system was not that the parties were too far apart. It was that they were too similar, and that they stood for too little. The parties operated as loose, big-tent coalitions of state and local parties, which made it hard to agree on much at a national level.
From the mid-1960s through the mid-’90s, American politics had something more like a four-party system, with liberal Democrats and conservative Republicans alongside liberal Republicans and conservative Democrats. Conservative Mississippi Democrats and liberal New York Democrats might have disagreed more than they agreed in Congress, but they could still get elected on local brands. You could have once said the same thing about liberal Vermont Republicans and conservative Kansas Republicans. Depending on the issue, different coalitions were possible, which allowed for the kind of fluid bargaining the constitutional system requires.
But that was before American politics became fully nationalized, a phenomenon that happened over several decades, powered in large part by a slow-moving post-civil-rights realignment of the two parties. National politics transformed from a compromise-oriented squabble over government spending into a zero-sum moral conflict over national culture and identity. As the conflict sharpened, the parties changed what they stood for. And as the parties changed, the conflict sharpened further. Liberal Republicans and conservative Democrats went extinct. The four-party system collapsed into just two parties.
The Democrats, the party of diversity and cosmopolitan values, came to dominate in cities but disappeared from the exurbs. And the Republicans, the party of traditional values and white, Christian identity, fled the cities and flourished in the exurbs. Partisan social bubbles began to grow, and congressional districts became more distinctly one party or the other. As a result, primaries, not general elections, determine the victor in many districts.
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.