There are three possible explanations. The first is that the Democratic establishment is spent. After the presidencies of Bill Clinton and Barack Obama and the defeat of Hillary Clinton, the traditional leadership of Democrats may have run out of ideas, energy and self-confidence.

This failure of nerve and imagination has left extremists in charge. Mr. Biden is like the French politician during the Revolution of 1848, who, perhaps apocryphally, said: “There go my people. I must find out where they are going so I can lead them.” The president is bowing to what he believes is inevitable—a party led by Mr. Sanders and the Squad, with him as the frontman. If you aren’t sure where this will end, look at what Jeremy Corbyn’s rise did to the British Labour Party.

A second explanation is that many traditional Democrats fear their rowdy left-wing base, which is big enough to win primaries and deep-blue congressional districts, but not purple or red territory. This may help explain the large number of Democrats in swing districts retiring from Congress.

A third explanation is that after eight years in the Obama White House being marginalized, ignored and treated as a kindly, lovable goofus, Mr. Biden likes being cheered by Democrats and media as potentially the most transformational president since FDR.