Similarly, while progressive organizations such as the Roosevelt Institute have developed fairly complex visions for strengthening regulation of Wall Street and banks and reducing the overall “financialization” of the economy, Mr. Sanders continued to fixate on restoring the Glass-Steagall Act, which separated investment and commercial banking but had become outdated long before it was repealed in 1999. His plan to “break up the big banks” sometimes seemed to consist simply of ordering the Federal Reserve to break up the big banks. The real progressive agenda has moved well beyond that to focus on raising and strengthening capital requirements, or the amounts that banks are required to keep as cash or safe investments.

Mr. Sanders made the $15 minimum wage a cornerstone of his campaign, probably accelerating the momentum that led to its passage in two states and the District of Columbia. But his campaign barely focused on other issues related to work, such as the challenges posed by new employment models in the on-demand, or “gig,” economy, a topic of a speech by Senator Elizabeth Warren in May. Ms. Warren called for a new social contract under which “all workers — no matter when they work, where they work, who they work for, whether they pick tomatoes or build rocket ships — should have some basic protections and be able to build some economic security for themselves and their families.”

This difference is part of a larger gap between Mr. Sanders and other progressives in their approaches to economic inequality. Where Mr. Sanders talks about “redistribution” of wealth from “the billionaires” to the middle- and low-income classes through high tax rates, others, such as the economists at the Economic Policy Institute, have focused more on what is sometimes called “predistribution,” wages and the conditions of work. They would reduce the gains at the top — such as by putting some meaningful constraints on executive pay — but also make sure that workers got a greater share of the profits, not only in the form of money, but also time, flexibility and predictable scheduling. If the initial distribution of benefits and money is badly skewed, it will be hard to use tax and transfer policies alone to redistribute it.

Mr. Sanders’s achievement in 2016 deserves respect: He has been the first insurgent Democratic candidate to emerge from the true left of the party since the Rev. Jesse Jackson’s two campaigns in the 1980s, and by far the most successful. That success points the way toward a new and more vigorous progressive agenda.

But he’s shown in his campaign that he’s unlikely to be the agent who fills in the details of that agenda. No doubt Senator Warren, with her ever-widening vision of economic fairness, will play a Kennedy-like role, whether she remains in the Senate or becomes Mrs. Clinton’s running mate.

Other Democratic senators, some almost as young as the Sanders enthusiasts, will play their part, as will outside organizations. If elected, Hillary Clinton will either join this new progressive wave or will be nudged and challenged by it. As Mr. Sanders finally steps back, the next era can begin.