The fallout from the release of Sen. Elizabeth Warren’s Medicare for All plan is the biggest event so far in the 2020 presidential campaign. It’s big enough that Sen. Warren’s campaign may have buckled beneath the weight of her plan’s fantastic details. But it might be bigger than that. The Warren meltdown could prove to be the Democratic left’s Chernobyl, a lasting catastrophe.

In 2016, capitalizing on socialist Bernie Sanders’s strong performance in the primaries against Hillary Clinton, the organized left muscled the Democratic...