Biden has trod lightly as he has sought to win over Sanders supporters, stressing his own personal relationship with the senator — a sharp contrast to Clinton's approach in 2016 — and being careful not to be seen as applying pressure to switch camps.

In a statement shortly after Sanders’ announcement, Biden made a personal appeal to his one-time rival’s base: “Bernie’s supporters: I know that I need to earn your votes. And I know that might take time.”

“But I want you to know that I see you, I hear you, and I understand the urgency of this moment. I hope you'll join us,” Biden wrote.

In recent weeks, Biden has taken some noticeable steps toward the left on certain issues: He’s backed a proposal from Sanders on student debt and one from Elizabeth Warren on free college, although Biden barely budged on his opposition to Medicare for All, a critical issue for progressives.

That could change, many progressives believe, as a wave of Americans are forced to seek treatment for the coronavirus pandemic. Already, more than 400,000 in the U.S. have contracted the virus, with many requiring hospitalization. Public support for single-payer health care surged in March, according to a Morning Consult/POLITICO poll.

Progressives also point out that Biden still lacks the kind of voter enthusiasm, particularly among young people, that has powered Sanders both politically and financially since launching his first presidential bid in 2015.

Biden’s campaign was already working behind the scenes to address the gap, reaching out to left-wing organizations like Indivisible and the climate-focused Sunrise Movement even as Sanders remained in the race.

Several of those groups, including Invisible, released a public letter to Biden on Wednesday laying out dozens of policy goals, pressing him to adopt the framework of the Green New Deal, to support Medicare or All and to support free tuition for public colleges.

But the wish list goes further: Progressives want Biden to support eliminating the filibuster and expanding the Supreme Court, to support the legalization of marijuana, and to appoint an attorney general committed to “dismantling ICE.”

Sanders’ decision to leave the race, effectively crowning Biden as the Democratic nominee, was a remarkably low fanfare occasion as the nation battles its worst health crisis in generations.

The fallout among members of Congress — many of whom had been fixated on the race in the months before the coronavirus outbreak — was equally muted. Few Democrats besides Sanders’ most loyal supporters were quick to weigh in on the race.

Unlike some of his earlier challengers, Sanders had always maintained a small group of congressional endorsements. Just eight Democrats in Congress had endorsed him.

Still, many lawmakers in the 100-plus member Congressional Progressive Caucus were rooting for Sanders and hoped he could secure policy concessions from Biden in the months leading up to what was once envisioned to be a contested convention.

Those hopes faded in recent weeks. First, after Biden trounced Sanders in stunning fashion on Super Tuesday on March 3, and then when the former vice president racked up more wins and solidified his lead a week later.

Since then, the focus of Sanders’ supporters in Congress has shifted to ways that the Vermont senator can exert his power at the convention and beyond.

Speaking on a video stream Wednesday, Sanders told supporters that he would actively apply pressure to the Biden campaign on his policy positions, calling Biden “a very decent man who I will work with to move our progressive ideals forward.”

And in a notable show of power, Sanders said he would keep his name on the ballot through the Democratic National Convention in a bid to influence key decisions, like the party platform.

“While Vice President Biden will be the nominee, we must continue working to assemble as many delegates as possible,” Sanders said.