WASHINGTON (AP) — Joe Biden on Thursday unveiled plans to expand eligibility for Medicare and forgive college debt for millions of Americans, as the former vice president begins courting progressives who lost their leader when Bernie Sanders left the presidential race.

Biden's swift move to shore up his left flank underscores the difficult choice many progressives were left after Sanders abandoned his presidential bid: Side with the more centrist former vice president or keep up the fight and potentially lose the White House again.

As they weigh their options, activists are already working to push Biden to the left on key issues. And he is amenable to the idea because he will likely need them to defeat President Donald Trump in November.

The first indication of just how much came when Biden announced he'd support expanding government-funded health insurance through Medicare to people 60 and older who opt out of health insurance sponsored by their employers — down from the current 65. Biden also said he'd back a plan to forgive student debt for many low- and middle-income borrowers.

Neither proposal goes as far as Sanders promised had he won the presidency. And they may not be enough to persuade supporters of the Vermont senator to embrace Biden.

“We can try all we want to use our leverage as a movement but, at the end of the day, I wouldn’t expect anything coming from the establishment, the Biden campaign or the Democratic National Committee as a way to bring in the base,” said Nomiki Konst, a Sanders surrogate who worked on party reforms on his behalf. “I think they want power — and I think they want money.”

Such sentiment could be a hurdle for a party desperate to defeat Trump. If Biden can't bridge the ideological divide, he risks heading into the fall with the same vulnerabilities as Hillary Clinton in 2016. But if he gives too much to progressives, he could be portrayed as a far-leftist, an argument the Trump campaign is already trying to make.

Despite Thursday's moves, Biden has signaled he's not willing to make major concessions, including embracing Medicare for All universal health insurance and the most sweeping Green New Deal policies to combat climate change.

Still, Biden has already embraced an overhaul of bankruptcy laws proposed by Massachusetts Sen. Elizabeth Warren, the other leading progressive, who ended her presidential bid last month.

RoseAnn Demoro, a close friend of Sanders and former head of the National Nurses United union, predicted Biden would make concessions on tuition-free college, labor and the environment, but not go as far as she and other activists wanted on health care. That leaves her unsure whether Sanders' supporters can be moved.

“The calculation is, this base has nowhere to go but Biden because of Trump," she said. "But if history teaches anything, a lot of the base sat it out last time.”

Demoro noted that, after 2016, many Sanders supporters knew he would try again for the presidency four years later. That seems unlikely going forward, potentially raising the profile of rising-star congressional progressives such as New York Rep. Alexandria Ocasio-Cortez, who endorsed Sanders, and Massachusetts Rep. Ayanna Pressley, who was a Warren backer.

Though he's suspended his campaign, Sanders' name will remain on the ballot in states that have not yet weighed in on the primary. He said Wednesday he still wants to collect delegates to influence the party platform.

Sanders and Warren have also notably stopped short of endorsing Biden for now.

Maurice Mitchell, national director of the Working Families Party, which backed Warren before shifting its support to Sanders when she dropped out, said Biden's goal is to rebuild the Obama coalition, which spanned generations, races and education levels. But he said Biden won't be able to do that without attracting the support of “young people committed to real, progressive change” who were most enthusiastic about Sanders.

“The question is, will Joe Biden increase voter turnout, be able achieve significant levels of voter enthusiasm, be able to achieve significant levels of individual volunteerism and small dollar donations and the type of enthusiastic voter to voter communication,” Mitchell said. “That is a political question that Joe Biden has to answer, and it can’t be done simply through rhetorical flourishes.”

Jennifer Epps-Addison, co-executive director of the Center for Popular Democracy Action, which endorsed Sanders, said progressives "are a real constituency” that Biden “will have to earn the votes of.”

“’Vote blue no matter what’ is absolutely not a winning electoral strategy,” Epps-Addison said. "Biden has some real negatives.”

The movement's next leaders might focus on building bridges with moderates rather than burning them — an approach Warren and Ocasio-Cortez have more closely adopted than some top Sanders supporters.

“Progressives have done a very effective job of moving the mainstream of the party in a more progressive direction,” said Sean McElwee, founder of Data for Progress, a data and messaging organization. “Many people in the Democratic Party have progressive sensibilities, and the way you win them over is you build relationships.”

Biden aides, meanwhile, began outreach to Sanders' camp to discuss policy weeks before the senator suspended his campaign. That included meeting with progressive leaders from at least two groups, the Sunrise Movement and March for Our Lives, who co-signed a letter Wednesday making certain demands of Biden if he hopes to win them over.

The former vice president himself, meanwhile, has had conversations with some of his former rivals — the kind of direct interactions that preceded his adopting Warren’s bankruptcy proposals.

Larry Cohen, chairman of Our Revolution, the offshoot of Sanders’ 2016 campaign, said he’d like to see the same kind of moves on other core issues for progressives. That could mean, instead of Biden building his “public option” health insurance plan as something only individuals can buy into, he could structure it so employers could buy in with their employees.

Whatever the outcome, Cohen argued that activist groups must stay aligned to maintain leverage. “The grassroots,” he said, “has to reach out together.”