“Many of the same politicians who now publicly embrace Trump privately dread him,” Nikki Haley said in February. “They know what a disaster he’s been and will continue to be for our party. They’re just too afraid to say it out loud. Well, I’m not afraid to say the hard truth out loud. I feel no need to kiss the ring.”

Yet there Haley was at the Republican National Convention in Milwaukee, kissing the ring. “President Trump asked me to speak to this convention in the name of unity,” she said. “I’ll start by making one thing perfectly clear: Donald Trump has my strong endorsement, period.”

Today’s Republican Party is what political scientists call a “personalist” party: It is built around a person, not an agenda or a coalition. The party is co-chaired by Trump’s daughter-in-law, Lara Trump. Dissidents like Liz Cheney and Chris Christie and Adam Kinzinger and even former Vice President Mike Pence have been exiled.

The Democratic Party has proved itself anything but a personalist party. Its convention this week will reflect a historic, collective act: the party’s mobilization to persuade its leader, Joe Biden, to step aside.

How have the two parties come to be such perfect opposites, photo negatives of each other? One reason predates Trump. The other reason is Trump.

There is a contradiction at the heart of the Republican Party that does not exist at the heart of the Democratic Party. Democrats are united in their belief that the government can, and should, act on behalf of the public. To be on the party’s far left is to believe the government should do much more. To be among its moderates is to believe it should do somewhat more. But all of the people elected as Democrats, from Representative Alexandria Ocasio-Cortez to Senator Joe Manchin, are there for the same reason: to use the power of the government to pursue their vision of the good. The divides are real and often bitter. But there is always room for negotiation because there is a fundamental commonality of purpose.