From the end of World War II to roughly the end of the Cold War, American politics and policymaking was markedly different. The Democratic and Republican parties were internally diverse, with distinct factions that often did not agree with each other on fundamental issues. Liberal Northern Democrats, such as Senators Ted Kennedy and Walter Mondale, had to contend with conservative Southern Democrats. In the Republican Party, conservative “Regular Republicans” had to contend with liberal Northeastern Republicans, like New York’s Jacob Javits and Massachusetts’s Edward Brooke (the “Rockefeller wing”). To pass anything, parties had to reach across the aisle to find willing partners from the other party—partners that changed depending on the issue.

On civil rights, for instance, to pass the landmark Civil Rights Act of 1964, Democratic President Lyndon Johnson relied on the votes of 25 Republican senators, including all 16 Northeastern Republicans, to end a filibuster by his fellow Southern Democrats. On economic policy, Republican President Ronald Reagan in 1981 relied on the votes of 50 “boll-weevil” conservative Southern Democrats to pass his budget—and later tax cuts—through a Democratic House of Representatives.

To many, this cross-party dealmaking was a sign that American politics was broken. The American Political Science Association called for “responsible” political parties who could present a coherent platform, ostensibly be elected on it, and then implement its planks. Others lamented the seemingly intractable divided government—in which Republicans reliably controlled the presidency and the Democrats controlled the Congress—of the 1970s and 1980s, going so far as to call for four-year House terms so that members of Congress were tied to the presidential ticket.

Beginning in the early 1990s, however, the political parties started to become more ideologically coherent and competitive, and the result was the broken policymaking we see today.

By the end of the century, conservative Southern Democrats and liberal Northeastern Republicans had become extinct. By 2001, not a single Republican member of the House was more liberal than any Democrat, nor a single Democratic member of the House more conservative than any Republican. As the University of Wisconsin-Madison political scientist Byron Shafer noted, by the Obama era, the number of House Republicans who were either closer to the ideological midpoint of the Democrats or to the ideological midpoint of Congress overall was essentially zero; for the Democrats, it was sub-10 percent. Both of these numbers were down from over half in the late 1950s.

Not only were the two parties more polarized than at any point since the late 19th century, they were also more balanced.

Consider that since 1992, the United States has had every single possible permutation of Democrats and Republicans controlling the House, Senate, and presidency. Contrast that with the stability of the years 1968 to 1992 in which Congress was controlled by the Democrats, and the presidency was—with one post-Watergate exception—controlled by the Republicans.