Bernie Sanders’s 2016 challenge to Hillary Clinton revealed a deep division in the Democratic Party—one that promises to get nastier as Joe Biden consolidates his status as 2020 front-runner.

The clash revolves around “electability,” which those of us on the left regard as a chimera. Centrists equate it to moderation, and they once had a point: Jimmy Carter and Bill Clinton both reached the White House as middle-of-the-road Southern governors while classic northern liberals like Walter Mondale, Michael Dukakis and John Kerry were defeated. But although Barack Obama was decidedly pro-business—Occupy Wall Street never forgave him for the bank bailouts—he marketed himself as a progressive via his hope-and-change rhetoric.

Today, centrism’s credibility lies dashed on the rocks of #ImWithHer. All Hillary’s dead-enders can do is insist that she “won the popular vote.” But she was supposed to win in a landslide. Some of her supporters blame recalcitrant progressives for her loss. They’re probably right.

Mrs. Clinton’s hawkishness and Wall Street ties were anathema to the left. After clinching the nomination, she pivoted right, even trying to create a Republicans for Hillary group. A stolen email from campaign chairman John Podesta listed Mr. Sanders 39th on a list of 39 vice-presidential prospects.

She shouldn’t have taken progressives for granted. More than 1 in 4 Sanders primary voters didn’t support Mrs. Clinton in November, according to a February 2019 FiveThirtyEight analysis of data from the Cooperative Congressional Election Study. Twelve percent voted for Donald Trump; the rest backed minor-party candidates or stayed home. That amounts to at least 800,000 votes—10 times Mr. Trump’s combined margins in the key states of Michigan, Pennsylvania and Wisconsin.