This is a relatively recent phenomenon. From the Great Depression until the Ronald Reagan years, the public had no problem with keeping Democrats in charge of Congress for decades, no matter which party held the White House. In those days, however, both parties were ideologically broad coalitions. Northeastern Republicans stood to the left of Southern Democrats, for example. Regardless of which party was in power, ideological diversity was assured, and like-minded politicians worked across party lines.

That changed, and changed in a big way, as the parties re-sorted themselves along ideological lines. Today, almost all Democrats are to the left of all Republicans. The result is that the system behaves very differently when one party is in control than when they share. So differently, in fact, that you can fairly say that the country has one Constitution with two distinct modes of operation.

In Mode 1  unified government  the minority party in Washington, shut out of power, has every incentive to make the majority’s life difficult, and does so. Its partisans, with no stake in whether anything gets done in Washington, treat the government as if it were under control of an invading army. The majority, lacking support from the out-of-power party, must govern on its own, which requires holding on to every element of its coalition, which means governing from the center of its party instead of the center of the country.

In other words, Democrats, when in total control, have little choice but to govern from the left. Republicans, who are even more conservative than Democrats are liberal, govern from the right. Policy is driven toward the edges, instead of the middle.

At that point, a third force comes into play: independents and moderates, who make up nearly a third of the country. As policy veers off center and partisan bomb-throwing escalates, they grow angry and disillusioned with what they perceive as an establishment that ignores their views and puts partisanship ahead of patriotism.