“First of all, Joe Biden knows what he’s doing. The places he’s reaching out are safe places,” said Jim Kessler, a longtime Biden backer with the centrist group Third Way.

“This race is not going to be won based on what a disgruntled Brooklyn coffeehouse denizen is thinking. It’s going to be won on turnout in cities and suburbs in places like Philadelphia, Milwaukee and Detroit,” Kessler said. “It’s time for Sanders to say to his supporters, ‘I’m with Joe. It’s time to go whole hog for him.’”

Sanders did not comment on Biden’s proposal, but campaign spokeswoman Briahna Joy Gray took a passing shot at Biden on Twitter.

“That he's willing to shift on these issues after telling us they were 'pie-in-the-sky' indicates that we can go further," she wrote. Gray said Biden should call for lowering the Medicare eligibility age to 55 immediately, and keep lowering it over four years until everyone is covered. In addition, "Tax Wall St & cancel ALL student debt," she added.

The cost of the policies Biden proposed is unclear, but likely big. The chances that a politically divided Congress would pass the legislation are slim.

But the ideas are likely popular, even with swing voters and independents, said veteran progressive activist Jeff Hauser.

“If Donald Trump wants to fight him over this, Biden’s stance should be, 'Bring it on,'” Hauser said. “Biden needs to not only commit to certain policies but convey authenticity in doing so.”

At the progressive group NextGen, which has already pledged $45 million to turn out young and minority voters in the general election, executive director Ben Wessel said Biden made “an important first step. But he needs more steps.”

“Biden has room to grow, and part of that is changing policy and showing that he’s listening to movement leaders, that he’s listening to young people, that he’s listening to progressives,” Wessel said. “Updating a PDF on a website, by itself, will not be enough to overcome the skepticism people have.”

