We often hear that both sides of politics benefit equally from polarization. This is plainly untrue.

Say what you will about President Trump, but he knows there’s only one way he can prevail. He needs to keep the nation deeply divided — by race, immigration status and religion, and by region, culture and ideology.

Trump’s rants seem — and often are — irrational, undisciplined and self-indulgent. But they are also shrewd and ­purposeful.

He accepts that he has permanently lost large parts of the country. But, given our electoral college, he knows he doesn’t have to win a plurality of the popular vote. He needs only tiny margins in swing states, and these require overwhelming support from whites, Christians, older Americans and people in small cities and rural areas.

He needs them to believe that he’s their champion and that his enemies — liberals and “socialists,” big-city folk and the “politically correct,” the secular and the culturally adventurous — hate them. If keeping that level of hostility high requires direct and indirect appeals to racism and xenophobia, he’s good with that.

If polarization helps Trump, then the opposite follows for progressives. They win only with coalitions that cross the lines of race, place and faith. Democratic candidates need strong support and turnout from African Americans, Latinos and city dwellers. But they cannot prevail in swing states without help from blue-collar and non-college-educated whites.

Moreover, the left and center-left believe that public action is a positive good, that social solidarity is a realistic possibility and that a society thrives when it shares benefits and burdens equitably. When we live in our bunkers of hatred, none of these dispositions has a chance.

It’s thus a big mistake for progressives to think that their own form of “base politics” is sufficient, and one politician who firmly grasps this is Sen. Sherrod Brown (D-Ohio). He has a lot to teach this year’s Democratic presidential candidates, and he gathered his thoughts in his delightful book, “Desk 88,” published last year.

Brown tells the stories of eight progressive senators who used the desk on the Senate floor that is now his own. They were, among other historic figures, Al Gore Sr., Robert F. Kennedy, George McGovern and future Supreme Court Justice Hugo Black.

But most revealing is Brown’s analysis of his 2018 reelection by seven points in a state Trump carried by eight. Rather than crow about his success in urban areas and college towns, he focuses on where he failed: “We lost medium-sized industrial city after medium-sized industrial city, small town after small town, rural community after rural community. Pretty much all of them.”

“Rural and small town voters don’t think either party is going to do anything for them, but they vote Republican because they think Democrats will do something to them: take their guns or raise their taxes, or enact an environmental law that will put them out of work,” he writes.

Bear in mind that Brown is no bland centrist. He’s a down-the-line progressive who campaigns hard for African American votes and speaks of linking labor rights to civil rights. His ideology, however, does not block his capacity for empathy. “I will never be one,” he writes, “who says that people in rural and small-town America vote against their own interest; who am I to say what is their self-interest? But we as progressives have work to do.”

It’s work essential not only to winning elections but also to creating a more humane and cohesive country.

I particularly appreciated Brown’s chapter on Kennedy, a politician known for his toughness but also, as his biographer Jack Newfield wrote, for his “almost literary ability to put himself inside other people, to see the world with the eyes of its casualties.”

Brown quotes these stunning words from Kennedy’s speech to the City Club of Cleveland during his 1968 presidential campaign, the day after the Rev. Martin Luther King Jr. was assassinated:

“When you teach a man to hate and fear his brother,” Kennedy said, “when you teach that he is a lesser man because of his color or his beliefs or the policies he pursues, when you teach that those who differ from you threaten your freedom or your job or your family, then you also learn to confront others not as fellow citizens, but as enemies — to be met not with cooperation but with conquest, to be subjugated and mastered.”

This is politics in the Age of Trump. We cannot let things go on this way.

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