The leaks from John Bolton’s forthcoming book are only the most recent revelations in the impeachment process. But despite all the revelations from current and former Trump officials, the chart of public support for removing Trump from office has been a flat line. Nothing changes people’s views.

To unpack those numbers more, consider this pattern. At the height of Watergate, the overall share of people who wanted to remove the president was not much different than it is today. In fact, days before Richard Nixon resigned, only 71 percent of Democrats supported his removal, compared with 89 percent who now support removing President Trump. Then, 55 percent of independents supported removal, similar to the 48 percent who do now. The biggest shift is with Republicans. In August 1974, 31 percent of Republicans favored Nixon’s removal. Today, only 8 percent of Republicans feel that way about Trump.

The story of this impeachment is the story of American politics today: polarization. It affects almost every aspect of American political life and has been studied by scholars from many different angles, with dozens of good historical and experimental approaches. Wouldn’t it be great if someone would digest all these studies, synthesize them and produce a readable book that makes sense of it all? Ezra Klein has done just that with his compelling new work, “Why We’re Polarized.” It is likely to become the political book of the year.

Klein begins by explaining that polarization is actually nothing new. Americans have been divided for a long time. The policy differences in the 1950s and 1960s between Southern segregationists and Northern liberals, or between free-market purists and Great Society advocates, were greater than those between most Republicans and Democrats today. But back then, each party contained a variety of political views, which meant these differences had to be navigated and negotiated. Liberal Democrats had to temper their zeal because their political power in the Senate depended on the segregationist Southern wing of the party. Since 1964, when the Democrats broke with the segregationists (obviously a good thing), the parties have sorted ideologically, and policy differences have become weaponized.

One mega shift that greatly exacerbated polarization is that partisanship today is largely about identity, not policy. And identity itself is increasingly determined by demographic factors — above all, after the Obama presidency, race. In one of the books Klein relies on, “Identity Crisis,” the authors point out: “Whites who did not attend college were evenly split between the two parties in Pew [Research Center] surveys conducted from 1992 to 2008. But by 2015, white voters who had a high school degree or less were 24 percentage points more Republican than Democratic.”

Once identities are at the heart of political differences, Klein argues, facts will not change minds. People have chosen their parties for reasons of tribal loyalty, and a better health-care bill will not alter their deep sense of belonging. This crucial insight is something Democrats in particular need to internalize. The key to gaining support among undecided voters probably lies in addressing their identity concerns rather than their economic ones. Past Democratic luminaries such as Bill Clinton were masters at this sort of symbolic politics.

Crucial in understanding tribal loyalty is negative polarization. Klein cites several studies that show that negative views of the opposing party are far more likely to get people to vote and contribute compared with positive views of their own party. He gives the example of former Texas representative Beto O’Rourke, who became a superstar when he ran against Ted Cruz for the Senate in 2018, but faded in his pursuit of the presidency last year because he did not have that negative galvanizer — opposition to Cruz — to fuel his campaign.

Because of the United States’ political geography, polarization affects the two parties differently, Klein argues. Republicans are a more homogeneous group, centered around white men, and have a huge geographic advantage, given the U.S. electoral system. Consider that they have lost the popular vote in four of the last five presidential elections and yet won the White House in two of those cases. Democrats need to appeal to a broader coalition than Republicans to compete in inland states and win the electoral college.

Klein’s book is powerful, intelligent and depressing. The American political system is not a parliamentary one in which one party gains control of all branches of government and can pursue its agenda. Power is shared among three branches, with overlapping authorities. The Framers despised the idea of parties and imagined constantly shifting factions. In their framework, some degree of compromise and cooperation is essential to getting anything done — which is why polarization has utterly paralyzed American government.

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