The concept is pretty simple: Over the past few decades, American politics has become like a bitter sports rivalry, in which the parties hang together mainly out of sheer hatred of the other team, rather than a shared sense of purpose. Republicans might not love the president, but they absolutely loathe his Democratic adversaries. And it’s also true of Democrats, who might be consumed by their internal feuds over foreign policy and the proper role of government were it not for Trump. Negative partisanship explains nearly everything in American politics today—from why Trump’s base is unlikely to abandon him even if, as he once said, he were to shoot someone on Fifth Avenue, to why it was so easy for vulnerable red-state Democrats to resist defecting on the health care bill.

Consider, for instance, that while Trump’s approval ratings have lately been in the mid- to upper 30s, he has maintained support of the overwhelming majority of Republican voters—around 80 percent in Gallup’s tracking poll. And that’s what matters to him and to most Republican members of Congress. The president understands that as long as that Republican base remains loyal to him, he is unlikely to face a serious challenge from GOP members of the House and Senate. He also knows that the surest way to keep the support of his base is by attacking Democrats, especially the two most prominent leaders of the Democratic Party—Hillary Clinton and Barack Obama.