It may be the most discussed fact about American politics today: The country is deeply polarized. The Republican Party has moved to the right by many measures, and the Democratic Party has moved to the left. Each party sees the other as an existential threat. One consequence of this polarization, politicians and pundits often say, is gridlock in Washington. But in a country that is supposed to have a gridlocked federal government, the past four years are hard to explain. These years have...