Yet they may come to regret leading the Democrats into a general election. If Mr. Sanders loses in November, there will be a replay of the vicious struggle for control that followed the 1972 defeat of George McGovern. Corporate centrists will argue, as they did then, that progressives produced defeat. All the misfortunes that befell the left after 1972—the “Third Way,” “triangulation,” Democratic Leadership Council centrism—emerged from that devastating loss to Richard Nixon.

The possibility that Mr. Sanders will become the first unreconstructed leftist to claim the presidential nomination of a major party in nearly half a century is as thrilling as a game of high-stakes poker. And it’s just as dangerous.

It may be shrewder for progressives to see Mr. Sanders lose the nomination to a more moderate sacrificial lamb like Pete Buttigieg or Amy Klobuchar —assuming that he or she goes down to defeat in November. A primary loss would allow the progressive movement to blame centrists for another Hillary-style catastrophe. Rather than lose to Donald Trump and strengthen the argument for business-as-usual Democratic centrism, progressivism would retain its theoretical electability.