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.

Kim McKinney Cohen, a leftist Democrat, told Politico her vote for Mr. Trump was “my primal scream” to the Democratic Party. “I wanted it burned down,” she said, “so that we could build a new, hopefully more equitable one that meets the needs of all, not only the superrich.”

Winning the next election isn’t necessarily more important than the long-term objective of winning over the Democratic Party. Progressives’ broader aim is to move the 50-yard-line of American politics to the left. In the Trump era they feel ascendant. The leftward surge in 2018 gave us Alexandria Ocasio-Cortez and other celebrity progressives. Most of the major Democratic presidential candidates—Mr. Biden notably excepted—at least pretend to be progressive by endorsing measures such as Medicare for All and the Green New Deal.