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
