A scrappy coalition of activist groups supporting Sen. Bernie Sanders plans to intensify its fundraising and organizing efforts against the Democratic establishment, which is coalescing around former vice president Joe Biden after recent presidential primary wins.

The sprawling network of socialists, climate-change activists, millennial organizers and other liberal advocates, who raise money from millions of members and have knocked on many doors in support of Sanders (I-Vt.), say they plan to cite the coronavirus outbreak and the uncertain economy to advance him as the only candidate who can enact broad economic and environmental change.

Their efforts come even as major Democratic super PACs line up to spend millions for Biden as the candidate they consider best positioned to win the party’s nomination.

Sanders, a prolific fundraiser in his own right among donors giving in smaller amounts, defied expectations to win early nominating contests before falling behind in delegates in recent weeks.

“It really looked like Bernie Sanders was winning, and now we’re back into something that’s a little more familiar: being the underdog,” said Alan Minsky, executive director of Progressive Democrats of America, one of the groups in the People Power for Bernie coalition. “The progressive left has not come this close to winning in America in my lifetime.”

Since Super Tuesday on March 3, the landscape of independent groups supporting the two candidates has changed significantly.

On Biden’s side, many of the party's wealthiest donors have embraced his candidacy after sitting on the sidelines or previously supporting other moderates in the primary. They are directing donations to the campaign and to the super PACs that raise and spend unlimited amounts to boost his chances.

These donors — including financier Donald Sussman, one of the party’s most generous givers, and billionaire Mike Bloomberg, who dropped out of the race — are newly supporting Biden and giving at an overwhelming clip, according to donors and fundraisers.

“I’ve never experienced anything like it,” said Alan Kessler, a longtime Biden donor and fundraiser, of the onslaught of donor interest. Fundraising for Biden “was a really tough slog two weeks ago. It’s not tough now.”

Meanwhile, People Power for Bernie said it is prepared for the long haul for the democratic socialist, who still has a narrow path to victory.

Among those in the coalition are activists working to elect socialists to city councils; housing advocates who want every person in the country to have access to affordable housing; youth activists rallying around combating climate change; protesters who camped outside the Florida governor’s office in the wake of Trayvon Martin’s death; and undocumented immigrants pushing for immigrant rights.

These groups have held events nationwide promoting Sanders, have filmed viral videos attacking moderate Democrats and have knocked on countless doors on behalf of Sanders. They are sharing talking points on Google Docs and stocking up on print fliers and snacks for door-knocking as they try to mobilize younger and minority voters for Sanders in upcoming contests.

Some activists are less optimistic. At least one group, the Center for Popular Democracy, said that it will remain supportive of Sanders’s campaign message but that it is shifting national messaging to be more focused on progressive values because it is “realistic about the challenges in the road ahead.”

“We always understood that the establishment was never going to be okay with someone as radical and progressive as Sen. Sanders to be the heir apparent to the Democratic nomination for president,” said Natalia Salgado, political director of Center for Popular Democracy.

In all, at least 16 groups unconnected to the campaign have spent money to promote Sanders by running ads and holding get-out-the-vote campaigns. These groups include the nine organizations in People Power for Bernie.

Although some of the groups in the coalition are newly formed and have little paper trail, others have existed for a few years and have cash to spend.

For instance, the Center for Popular Democracy had about $22.6 million in 2017, according to the group’s latest tax filing. People’s Action, a nonprofit organization, received $7 million in grants in 2017.

It is unknown how much most of these groups have raised to support Sanders in the 2020 cycle or where they are getting their money because they are not required to disclose their donor lists. Advocates for more transparency in political donations — predominantly on the left — refer to this type of funding as “dark money.”

The groups reject that label, saying they have internal measures to vet the sources of their donations so that they are not taking money from billionaires or special interests that would undermine their mission. They said they are respecting their donors’ desire to remain private.

“Being an openly socialist organization, for us, it feels very important to protect the privacy of our members while also making it very clear that we don’t accept corporate funding,” said Sean Estelle, one of the leaders of the Democratic Socialists of America’s national campaign to support Sanders.

On Biden’s side are some of the most well-funded super PAC operations, whose donors are made public. These groups have spent more than $9.8 million to support Biden, mainly through Unite the Country, a super PAC started by Biden’s allies.

Bloomberg, who dropped out of the presidential race after Super Tuesday, is starting a group to run independent expenditures to support Biden. On Wednesday, Bloomberg’s campaign published copies of its anti-Trump ads for public use — worth $275 million in ad spending — as the campaign shifts the billionaire’s resources to help defeat President Trump in November.

Priorities USA Action, the main pro-Democratic super PAC, is shifting its advertising messaging to defend Biden — a sign that the establishment considers him the likely nominee.

American Bridge, another major Democratic super PAC, on Wednesday also shifted its general-election messaging to support Biden. It began running $2.2  million in ads in Pennsylvania, aimed at boosting Biden through television, radio and digital ads.

“The voices of Democratic voters are loud and clear: they want Joe Biden to be our standard-bearer,” American Bridge President Bradley Beychok said in a statement. “That’s why we are rallying behind his candidacy and will deploy every resource at our disposal from now until November to ensure Joe Biden is the next President of the United States.”