If the moderators are skilled, they might be able to suss out some real contrasts in the contenders’ views and approach to governance.

The first night features, among others, Sen. Elizabeth Warren (D-Mass.) — an advocate of Medicare-for-all — and Sen. Amy Klobuchar (D-Minn.), an advocate for expanding access under the Affordable Care Act and addressing high drug prices. Moderators should ask each what’s wrong with the other’s approach, and ask them both how they get past recalcitrant Republicans. Warren wants free college, and Klobuchar says no way.

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Beto O’Rourke has one of the skimpiest records, while Washington Gov. Jay Inslee (as a congressman and two-term governor) has one of the most impressive. O’Rourke could be asked to explain why he thinks Inslee, who has done many of the things O’Rourke talks about (universal access to health care, education reform, green-energy initiatives) isn’t a better choice; Inslee could explain why his green-energy plan is stronger than O’Rourke’s.

Night two will have onstage four of the heaviest hitters (former vice president Joe Biden; South Bend, Ind., Mayor Pete Buttigieg; and Sens. Bernie Sanders of Vermont and Kamala D. Harris of California). There will be plenty of opportunities to reveal the contenders’ strengths and weaknesses. Moreover, it’s a rare chance to get candidates to confront one another on areas of disagreement.

Biden voted for the Iraq War. Why should we trust the judgment of Biden over that of Buttigieg, who was deployed to Afghanistan for seven months in 2014?

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Biden and Sanders should each explain why it’s a good idea to elect a septuagenarian. Will they release their complete health records by the end of the year? Buttigieg should be asked to be candid: Isn’t there a lot he doesn’t know that he’d have to learn on the job?

Sanders has achieved a fraction of what Biden has accomplished. Why choose someone who’s never delivered on his radical agenda over someone who’s made significant, albeit limited, progress on a range of fronts?

Buttigieg defends capitalism but acknowledges its imperfections. Let him and Sanders make the case for democratic capitalism and democratic socialism, respectively. (Will Buttigieg remind Sanders that Franklin D. Roosevelt thought he was saving capitalism and hence is not the socialist role model Sanders paints him to be?)

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Harris and Biden should go back and forth on the 1994 crime bill. Was it a necessary response to rampant crime with unintended consequences or a misguided effort that did more harm than good?

What does Biden know from his years in the Senate and the Obama administration that the others don’t? What does Buttigieg know from his time in the military that the others miss?

On some issues, most of the 20 candidates say the same thing (“end long wars,” “stand up to China without hurting Americans”) but don’t say how they’d do it. Each one should be pushed to explain how they would end wars without giving terrorists safe havens. If a tariff war isn’t working against China, should we, for example, revive the Trans-Pacific Partnership, which put China at a political and economic disadvantage in Asia? On repairing the damage Trump has done to our constitutional democracy, ask them to identify a few reforms for the Justice Department and for intelligence oversight.

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The idea should be to let the candidates do most of the talking and, gosh, maybe even debate one another now and then.