WASHINGTON — Progressives are worried the White House could be slipping away. They've been there before.

For a moment, 2020 looked like the year the left was finally going to seize power, via either Bernie Sanders or Elizabeth Warren. But Warren is gone as of Thursday, and while Sanders could still win the Democratic nomination, he's facing another long slog against the newly united party establishment after missing a chance for an early knockout on Super Tuesday.

In other words, it's back to usual.

"When has the left ever easily won in this country?" asked Waleed Shahid, a spokesperson for the left-wing group Justice Democrats.

Joe Biden's sweep through Super Tuesday left progressives stung, with most preparing to back Sanders, some looking to pressure Biden and others already gazing to a post-Sanders era of progressive politics.

For most on the left, however, supporting Sanders will now be their immediate focus — and they hope an influx of Warren supporters will give Sanders a jolt of momentum.

But some worry that another long and bitter primary might hurt whoever emerges as the nominee and help President Donald Trump win re-election, which will make it incumbent on both Sanders and Biden to do a better job of welcoming the other's supporters than Sanders and Hillary Clinton did four years ago.

"Sometimes, it feels like 2016 never really ended. I was hoping for a new beginning, but maybe this is just the sequel," said Rebecca Katz, a progressive Democratic strategist. "If we are going to beat Donald Trump, we're going to have to come together on some level, but there is a real frustration on all sides about the lack of understanding on the other side."

"There is one progressive candidate left in this race, and that's Bernie Sanders," Ari Rabin-Havt, Sanders' deputy campaign manager said on MSNBC. "If people want a progressive vision of the future, we invite them to come into our campaign to support that progressive vision."

Warren, though, has not yet endorsed Sanders or anyone else, and polls of Warren voters, who tend to be better educated and wealthier than Sanders' base, suggest that not all of them will go to him despite their ideological agreement.

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Privately, some Warren allies say she might be able to exert more influence over Biden's campaign and potential administration than Sanders since her support could be more valuable to a moderate candidate.

Sean McElwee, the co-founder of Data for Progress, whose polls have largely been accurate in the 2020 primary, estimated that while Sanders would get a majority of Warren's supporters, Biden could still earn up to 40 percent of them.

"Warren supporters can help Bernie with college-educated suburban women, a problem demographic for him," McElwee said. "At this point, he really needs anything he can get as we go into some pretty tough terrain."

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More than just leaving Sanders behind in the delegate count, Super Tuesday was the first large-scale real-world test of his strategy for winning both the primary and the general election, and it came up short.

He promised that his political revolution would turn out a multiracial coalition of working-class voters and young people. But with the notable exception of Latinos, it was actually Biden who tended to win working-class white and black voters and increase turnout in battleground states like Virginia and North Carolina, while turnout among young people was not huge either.

There were disappointments further down the ballot for the left as well. Moderate Rep. Henry Cuellar, D-Texas, narrowly hung onto his seat despite a high-profile challenge from a young progressive Latina backed by both Sanders and Warren.

The liberal media personality Cenk Uygur, whom Sanders endorsed and then quickly un-endorsed, has so far earned just 6 percent of the vote in a primary to fill the seat vacated by former Rep. Katie Hill in California. And Washington Democrats' favored Senate candidates in Texas finished strongest.

Still, Biden retains all the weaknesses that made Democratic elites reluctant to consolidate around him in the first place, and Sanders still remains popular. This race has changed quickly before and could again, especially heading into a two-person contest when voters tend to allow candidates to draw sharper contrasts with each other.

"If Bernie posts some good fundraising numbers, notches a high-profile endorsement and sharpens his attacks on Biden's record, it's not hard to imagine the momentum next week swinging to his favor," said Karthik Ganapathy, a former spokesperson for Sanders' 2016 campaign.

Sanders is already beginning to retool his strategy and will need to broaden his base.

"Most of the voters that he needs to bring into his fold are ordinary democrats who don't identify as radical or even left and who are largely concerned about beating Trump," David Segal, a progressive strategist and former state legislator in Rhode Island.

Sanders can do that without sacrificing his values, Segal said, by emphasizing that his record as a legislator is actually far more pragmatic that he typically lets on and that his agenda is now, five years after he upended the party, not that far from the mainstream of contemporary Democratic politics.

For instance, a majority of Democrats in every state where NBC News exit polls were conducted on Super Tuesday said they favor "replacing all private health insurance with a single government plan for everyone." In Texas, "Medicare for All" was favored by 63 percent.

"On a daily basis the campaign still calls for a political revolution — and then also explains why its program is not, in fact, revolutionary. This isn't coherent," Segal said.

But some are already looking ahead to a post-Sanders progressive movement.

The Vermont independent brought new energy and people into politics, but also, critics say, made it more difficult for a movement to build beyond him, and he has yet to prove he can build a winning national coalition.

Warren has long been favored by the professional activist class of the Democratic Party, and it was the Massachusetts senator, not Sanders, whom they initially tried to draft to run against Clinton in 2016.

"Bernie does nothing for the progressive movement," said Markos Moulitsas, the founder of the progressive blog DailyKos, who has been critical of Sanders.

"So what do progressives do? Keep building capacity at the state level," he added. "Biden isn't our answer, but we have to stop thinking of the presidency as the only game in town."