After Barack Obama’s belated endorsement of Kamala Harris, Bernie Sanders is now the only prominent politician on the left side of the spectrum yet to have endorsed Harris’s nascent presidential bid. Sanders’s endorsement is likely forthcoming, but the fact that it hasn’t happened already is not simply a matter of political timing the way Obama’s was. In an interview with Anderson Cooper, Sanders claimed he would seek assurances from Harris that she supports policies to fight plutocracy and to help working families survive an economy increasingly hostile to them. In the same interview, Sanders expressed continued discontent with the way Biden was forced out of the race—and particularly with the role that the media and major donors played.

But even so, Sanders’s support for Biden was puzzling. In the face of undeniable evidence, Sanders and other members of the progressive left were backing a candidate who was clearly unfit to serve another term, and who was well behind Trump in the states he would have needed to win. Then there’s Biden’s unconscionable support for Israel’s barbaric attacks on Palestinian civilians, which Sanders has criticized vociferously. Given the stakes and the candidate, Sanders’s support was irresponsible. And as Biden’s decision to “stand down” became inevitable, Sanders’s support also seemed like a strategic blunder, making him and the left wing of the Democratic congressional caucus look weak.

New reporting has shed further light on Sanders’s dead-end allegiance to Biden—the very man who marshalled the support of donors, mainstream media, and establishment actors to beat him in the 2020 primary. As Biden lost support among mainstream Democrats and donors in recent weeks, he pivoted left both publicly and privately. Biden administration officials floated Supreme Court reforms, including term limits and enforceable ethics rules, to make that radically conservative institution more accountable. Biden also promised rent-caps, higher taxes on the wealthy, medical debt relief, and an expansion of Social Security.

Perhaps Biden had become convinced of what Sanders and others have long been saying: that Democrats needed more vibrant pro-worker, anti-corporate, and anti-corruption policies and messaging to beat Trump. Simply calling Trump a threat to democracy, true though that may be, wasn’t doing the trick. Or perhaps Biden was simply desperate to shore up support on his left as he hemorrhaged support from his party’s center.

In any case, Sanders saw an opportunity in Biden’s weakness to push him further left. There’s some plausibility to the gambit. Sticking with Biden was a low-risk, high-reward maneuver. Sanders has lost face temporarily, but not much else. But were Biden to have stayed in the race and somehow won, Sanders and the left would have secured increased influence over Biden’s second term, during which Biden would also have been alienated from centrist Democrats and—without another election to face—freed of the need to appease them.

But that calculation doesn’t come close to cinching the case. After all, Biden’s promises were made by a candidate almost certain to lose and put Trump back in office. Sanders was tying himself to a sinking ship.

Nevertheless, his decision makes a kind of sense if he also regarded the end of the Biden presidency as the possible end of an era of relatively strong left-wing influence on domestic policy. What made this influence possible was not Biden’s own personal convictions. In the Senate Biden earned the nickname “Credit Card Joe” for votes and advocacy that shielded banks and corporations from scrutiny, and helped to protect Delaware’s status as a tax and legal sanctuary for major corporations. There’s little to suggest Biden underwent some grand ideological change of heart just as he was taking office in his seventy-ninth year.

The reasons for his administration’s relatively progressive domestic agenda have more to do with Sanders’s strong showings in the 2016 and 2020 Democratic primaries, and the forces that led to, and were amplified by, these showings. They moved the party left and established Sanders and other economically progressive politicians like Elizabeth Warren as major influences in the Democratic Party. Along with the consensus that Obama’s post-2008 economic stimulus had been too small and the special challenges presented by the pandemic, the left’s success in the primaries led to the worthy, if limited, legislative accomplishments of Biden’s first term: in addition to the Covid stimulus, infrastructure and climate investment, antitrust enforcement, and pro-worker industrial policy.

But as Sanders’s electoral successes (such as they were) and the crises occasioned by Covid fade into distant memory, it’s uncertain whether the left’s influence will last long enough for the Democratic Party to complete its turn away from neoliberalism.

Sanders is unlikely to have the same relationship with a President Kamala Harris, and other structural factors are holding back the growth of real leftwing power. The chances of building a popular movement from the bottom up are hamstrung by a persistent disconnect between the labor movement, which remains weak despite recent organizing successes, and the political and intellectual left—a large faction of which has taken dogmatic cultural positions that make building coalitions with working-class voters difficult. No progress has been made on reducing the influence of money on elections, as recent controversies over Harris’s connections to the tech industry have reminded us. And the Democratic establishment has shown itself increasingly willing to manipulate—or even sideline—the primary-election system that elevated the left in the first place and ignited productive intraparty arguments over health care and the economy.

It would be no surprise, then, if Sanders saw in the effort to oust Biden the risk of firmer donor and corporate control of the Democratic Party. It’s understandable that, given the weakness of the left, he preferred a candidate he could influence over a replacement candidate likely to be more beholden to monied interests. That he did so in the face of the public’s clear wishes and Biden’s clear incapacity—and with Trump waiting in the wings— speaks ill of his judgment. But it also may speak to a deep pessimism, bordering on desperation, about the power of the left and prospects of the Democratic Party, which, despite its vigorous resistance to Trump the man, has shown only a very limited willingness to resist the political and social trends behind his rise. In the longer run, if no powerful political movement is capable of combatting plutocracy and promoting real democracy, the country will face problems even grimmer than a second Trump term.