What’s the best way to ease tensions in a divided republic? Address them squarely in a national election.

Donald Trump, longtime businessman, versus Bernie Sanders, lifelong socialist, would give us that once-in-a-lifetime political contest. Both are larger-than-life personalities whose virtues and drawbacks are well known. The other Democrats running for president may differ from Mr. Sanders only marginally on many issues, but he alone genuinely personifies leftist orthodoxy. That’s why he’s still standing while so many...