The 2024 election is promising to be one for the ages, and an underplayed story so far is how the left wing of the Democratic Party has so easily fallen in behind President Biden. Far from challenging Mr. Biden, Bernie Sanders has emerged as one of the President’s most vociferous supporters.

The Vermont socialist has been hitting the airwaves and hustings to tell progressive voters to back the President. He’s trashed the idea of a third-party run by the black left-wing professor Cornel West. You might even say Mr. Sanders is Mr. Biden’s leading surrogate, and no doubt he’s acting with the blessing of the White House.

“President Biden and his administration have made some real progress in addressing issues that have not been dealt with in decades,” Mr. Sanders said Sunday on CBS ’s Face the Nation. He cited drug price controls and public-works projects in particular before pivoting to say that Democrats have to do much more to win back the “working class.”

He said Mr. Biden has to campaign for “real Democrats, not corporate Democrats, like (Joe) Manchin and (Kyrsten) Sinema,” so he can get full control of Congress. “I think if he runs on a strong progressive agenda, he’s not only going to win, he’s going to win by a strong vote.”

In other words, Mr. Sanders is betting on a second Biden-Bernie administration. In 2020 Mr. Sanders lost to Mr. Biden in the primaries but he managed to win from Mr. Biden a joint policy task force that embraced much of the Sanders-Elizabeth Warren agenda. And Mr. Biden has governed that way, embracing student-loan forgiveness, vast new climate regulation, and record federal spending, among other progressive priorities.

Ron Klain, the former White House chief of staff, made the calculation that Mr. Biden needed to unite Democrats as President, rather than uniting the country as he promised as a candidate. Now Bernie’s full-throated endorsement is important evidence of what Biden II (or Biden I1/2 and Kamala Harris I) would look like.

This is the reason Mr. Biden faces no serious Democratic challenger despite his manifest decline and anemic popularity. But the results of that agenda are the reason Mr. Biden is vulnerable to a Republican, especially anyone other than Donald Trump.

This editorial first appeared in The Wall Street Journal on September 6, 2023. It was distributed by The Associated Press.