Many statistics point to the same trend. The frequency with which the House and Senate flip between parties has increased; neither party builds much momentum for long. Democrats controlled the House from 1955 to 1995, for instance. Now the chamber is about to change hands for the fourth time in 15 years. The difference in the number of congressional seats held by the majority and minority parties has collapsed too. Between 1959 and 1995, the House majority was never less than 50 seats and repeatedly hit the triple digits. Today, the Democrats have only eight more seats than the Republicans. The popular-vote margin in American presidential elections has diminished too. In five of the past six contests, fewer than five percentage points separated the winner from the loser.
Every election is close. Every election feels consequential, because it is. And far-reaching policy outcomes are over and over again determined by just a few thousand votes in a handful of states.
In many ways, this is a baleful trend. Our coin-toss elections are not the result of having two parties competing for an engaged and persuadable electorate. They are at least in part a product of our political stasis and extreme polarization. They mean that when either party wins, it does so without much of a mandate. They also mean that neither party is ever forced to regroup and reform after a humiliating defeat. In the words of the Princeton political scientist Frances Lee, both parties are “insecure” in victory and hyper-engaged in a “perpetual campaign” against the other side.
There is no simple explanation for why this is happening. I had at first figured it had something to do with game theory. You have two parties. Each has deep resources and strong incentives to win over and thus reduce the number of middle-of-the road voters. The parties zig, they zag, they revise their arguments. They both get better at winning elections, beefing up their use of voter data, opposition research, on-the-ground organizing, and gerrymandering. In time, each gets good enough that the contests start to come out 50–50.
That hypothesis might be partly true, John Sides, a political scientist at Vanderbilt University, told me. But he pointed to historical factors that, in his view, offer the strongest explanation for today’s political environment. During the Great Depression, the Democrats became a nearly unshakable majority party in Congress, buttressed by the votes of white southerners and New Deal supporters across the country. But in the 1960s, white southerners began migrating to the Republican side in a revolt against national Democrats, who backed civil-rights legislation and redistributive policies that aided Black Americans. As the political saliency of the New Deal faded, the parties became more competitive and their voter bases more equal. “The thing that made us so unusual for so long was the South,” Sides said. “We had a one-party state within the country.” Its disappearance paved the way for our 50–50 electorate and pendulum-swinging government.