Early on in his newly released Why We’re Polarized, Vox cofounder Ezra Klein explains that his book is not meant to guide us out of the morass that is American politics. Instead, he hopes to deliver a helicopter view showing us how we got so deep in, to view from above the system driving us all batty.

“What I am trying to develop here isn’t so much an answer for the problems of American politics as a framework for understanding them,” Klein, a self-acknowledged liberal, writes in his introduction. “If I’ve done my job well, this book will offer a model that helps make sense of an era in American politics that can seem senseless.”

For Klein, that making sense can only begin if you accept that where we are today in terms of partisanship is fundamentally different from where we’ve been in the past. “The first thing I need to do is convince you something has changed,” he writes. What has changed, exactly, is that “the Democratic and Republican parties of today are not like the Democratic and Republican parties of yesteryear.” Namely, in the past, being a Republican or a Democrat was “not a rich signifier of principles and perspectives.”

For instance, Klein cites the example that, in 1954, Minnesota’s very liberal Hubert Humphrey and South Carolina’s extremely conservative segregationist were both Democrats. Once upon a time, it was more common for voters to split their tickets. Voting for a Democratic president and voting for a Republican senator were not mutually exclusive actions. To that end, Klein highlights an analysis by political scientists that examined contested House races throughout the past half-century. “Between 1972 and 1980, the correlation between the Democratic share of the House vote and the Democratic share of the presidential vote was .54… By 2018, it had reached .97!” Somewhere along the way, we switched from having a level of comfort with the opposing party to not trusting it at all, to thinking that voting for someone on the other ticket was, essentially, an act of treason.

Why this realignment happened is complicated—and much of it has to do with the Civil Rights Act of 1964 and the split it caused in the Democratic Party between Northern liberals and Southern conservative Dixiecrats.

“What has happened to American politics in recent decades is that the parties have become visibly, undeniably more different, and the country has become more partisan in response,” Klein writes of this realignment. “Across 10 measures that Pew Research Center has tracked on the same surveys since 1994, the average partisan gap has increased from 15 percentage points to 36 points.” There have always been differing views on everything from immigration to health care in this country, but those views were not as trenchantly sorted across party lines as they are now. In years past, there were more elected Republicans who were pro-choice (former Wyoming senator Alan Simpson) or pro-immigration (former Florida governor Jeb Bush) or pro-environment (former New Jersey governor Christine Todd Whitman), and Democrats who supported school segregation (former vice president Joe Biden in a previous era). When Maine senator Olympia Snowe, a pro-choice moderate, shocked colleagues with her retirement, citing “partisanship of recent years in the Senate,” one Republican aide opined: “I think she just doesn’t fit this place anymore.” Or consider a rather jarring quote Klein pulls from a former president: “I believe in the idea of amnesty for those who have put down roots and who have lived here even though sometime back they may have entered illegally.” Who said that? Ronald Reagan. Today, that type of policy cross-pollination seems unthinkable.