At Thursday’s Democratic presidential debate, distinctions among the candidates were rooted in different theories of change. The party’s left flank argued that Americans will move in its direction if provided with unapologetic leadership. The relative moderates in the race hewed more closely to where Americans actually are. One path enhances the likelihood of President Trump’s reelection, with all of the damage that implies. The other could lead to responsible government.

Sen. Elizabeth Warren (Mass.) defended her plan to impose the largest tax increase since World War II, Medicare-for-all, free college for all and other massive social programs, arguing that only Washington corruption stands in the way. “Attack the corruption head-on,” she urged, and then Congress will act.

Sen. Bernie Sanders (I-Vt.) said Democrats should not aim to reach out to the center but to attract “the largest voter turnout in the history of America.” He explained: “You don’t have the largest voter turnout unless you create energy and excitement.” Naturally, he said his specific democratic socialist agenda would create that energy.

But one can object to Ms. Warren’s program without being corrupt, and one can look at opinion polling to see that Mr. Sanders’s agenda would scare many Americans, even as it excites some. Their agendas probably would fail at the polls and, if not, would carry extreme risks if they tried to implement them.

By contrast, South Bend, Ind., Mayor Pete Buttigieg implored Democrats to “be smart about the promises we’re making, make sure they’re promises that we can keep without the kind of taxation that economists tell us could hurt the economy.” He deplored the “mind-set that measures the bigness of an idea by how many trillions of dollars it adds to the budget or the boldness of an idea by how many fellow Americans it can antagonize.”

Sen. Amy Klobuchar (Minn.) said Democrats should stick to the strategy that has proved effective election after election: building a coalition of “moderate Republicans and independents, as well as a fired-up Democratic base,” as she did in her Midwestern home state. This does not mean one must sell out one’s principles, she argued, in part because there are many ways to achieve, say, universal health care or college access: “You can be progressive and practical at the same time,” she said.

Democrats must get this right because, as former vice president Joe Biden argued, there can be — in U.S.-style democracy, there must be — cooperation amid legitimate disagreement. “I refuse to accept the notion, as some on this stage do, that we can never, never get to a place where we have cooperation again. If that’s the case, we’re dead as a country.” Democrats can be clear-eyed about the current state of the Republican Party without cheering for further descent into partisanship and polarization.

A “healthy American majority exists for common-sense positions that are also progressive positions,” Mr. Buttigieg said to us in a recent interview. “Whether it’s on climate, whether it’s on health, on wages, even issues where my party has been on defense, like immigration or guns. . . . The job of a nominee, and certainly the job of the president, is to galvanize and not polarize that majority.”

In a diverse democracy, the alternative to unifying and coalition-building is almost never winning a grand, definitive victory that changes everything — a “political revolution.” It is gridlock and failure. Some Democratic candidates are offering a more positive future.

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