Sen. Bernie Sanders (I-Vt.), a Democratic presidential candidate, addresses the crowd at Netroots Nation, an annual gathering of liberal activists, at the Phoenix Convention Center. (Charlie Leight/Getty Images)

First came the People for Bernie reception, held by Occupy Wall Street veterans, at a club modeled after a Venetian estate. One night later came the We Want Bernie party, hosted by Progressive Democrats of America in a low-key taqueria.

And on the third day, Bernie Sanders himself appeared — and named what many of the liberal activists who gathered here for a conference last week consider the original sin of the Obama presidency.

“Today, the largest six financial institutions in this country have assets of some $10 trillion, equivalent to 60 percent of the GDP of America,” the senator from Vermont told a crowd of 11,000. “After we bailed them out, because they were ‘too big to fail,’ most of them are now a lot bigger than they were before.”

The surge of Sanders’s presidential campaign has been widely viewed as a gambit, by liberals, to force Democratic front-runner Hillary Rodham Clinton to the left.

But many liberal activists see no other way for Clinton to win. They are not just supporting Sanders’s agenda, or asking Clinton to embrace free college tuition or vast new infrastructure spending. They are telling an alternative history of modern liberalism.

Sen. Bernie Sanders (I-Vt.), a White House contender in 2016, is known for his stances on budget issues and war. Here are his takes on Obamacare, Social Security and more. (Julie Percha/The Washington Post)

To these activists, it was the Democrats’ failure to prosecute Wall Street after the 2008 crash that enabled the tea party movement and the Republican Party’s comeback in Congress. It was the kludged, mandate-driven design of the Affordable Care Act that prevented it from being a boon to Democrats, as Medicare for all might have been.

And Sanders has carried that story into the primaries. In a February speech, he bemoaned how President Obama had “missed the opportunity, politically, of doing what [Franklin] Roosevelt did when he was elected and making it clear to the American people what is happening and why.” In an interview last year with Bloomberg News, he said that “the key mistake of the Obama administration, starting from the day after he was elected, was to more or less disband the grass-roots network that he had put together to get elected.”

Sanders’s campaign, and its allies, are pledging never to repeat that. In late June, the grass-roots group Ready for Warren gave up on drafting Sen. Elizabeth Warren (D-Mass.) to run for president and rebranded itself as the pro-Sanders Ready to Fight. It joined the ranks of Progressive Democrats of America — whose founder, Tim Carpenter, died in 2014 after beseeching Sanders to run — and the Occupy-colored People for Bernie.

Some of the best-attended panels at last week’s Netroots Nation conference here dealt with how activists could bypass the mainstream media and make a bad bill or bad law or bad police tactic too infamous to continue. Some of the loudest applause came when Warren recounted how progressives had campaigned against some Obama administration appointees.

“As soon as [Obama] got elected, they should not have held back their pressure,” said Netroots Nation participant Felipe Andres Coronel, a rapper best known by his stage name, Immortal Technique. “We should have put the pressure on and made it even harder. There were people who made their presence known. Wall Street made its presence known.”

Outside of Netroots Nation, among the Democrats who had followed the Obama strategy, these critiques sounded part cruel and all wrong.

“Look, I wish this country were further left than it is, but I live in reality,” former congressman Barney Frank, a Massachusetts Democrat who worked with the White House to overhaul Wall Street regulations, said in a phone interview. “These people watch MSNBC, and they talk to each other on the Internet — in some ways, they’re like the tea party. They’re in this parallel universe.”

David Axelrod, the Obama campaign strategist who was a senior White House adviser during the crisis years, said Sanders-style criticism ignores the reality of the economic crash. “There was a constant tug and pull between the need to discipline an industry whose excesses had triggered the crisis and keeping the system from collapsing, which would have exacerbated an already reeling economy,” he said. “Senators don’t face such conundrums. Presidents do.”

To the Netroots activists, the problem was that the Obama administration considered this a “conundrum” at all. Going after the bankers, they say, would not have crashed the economy, and it was not their fault that Obama hired people who failed to understand that. Now, all they can do is try to ensure that the next Democrat hires the right people.

When the doors opened here for Sanders’s speech on Saturday morning, those who had been waiting in line sprinted to get as close as possible to the stage.

Sanders and Warren “are saying the sorts of things — like Jon Stewart is saying the sorts of things — we wish we heard everywhere,” said Connie Aglione, 67, an Arizona activist.

Athena Soules, 36, an artist who designed banners for the Occupy Wall Street movement, said Sanders’s success is possible in part because of the energy generated by the 2011 protests that began in Lower Manhattan. She unfurled a banner — “Feel the Bern,” it read — surrounded by fellow activists.

“We learned the hard way from Obama,” Soules said.

Said Katherine Brezler, 33, the national digital organizer for People for Bernie: “We elected Barack Obama on the idea that we were going to get a universal health-care system. We have 35 million people uninsured in this country.”

Pressuring politicians to the left was the original idea when Netroots Nation was founded in 2006 by readers of the Daily Kos group blog. The conference (originally “Yearly Kos”) matched Democratic candidates with a growing, active left. In 2007, every major presidential candidate participated in a forum, where Clinton said that some lobbyists “represent real Americans” — a quote that haunted her. Her 2008 primary defeat was celebrated as a victory of activist power over a tired establishment.

In 2011, the White House dispatched senior adviser Dan Pfeif­fer to the conference for a bristling audience Q&A. This year, there was no White House presence to speak of, and that was welcome. There was no visible organizing for Clinton — who skipped the conference — and that was barely noticed unless reporters asked about it.

“The way that the Obama victory sucked out all the energy of progressive politics in the United States — it harmed us across the board,” said Charles Lenchner, 45, an activist who had campaigned to get Warren into the presidential race. “What a terrible time that was.”

Lenchner cited the collapse of ACORN, a community group that lost congressional funding after a conservative video sting, as a nadir of the Obama years. For some, the lowlights started earlier. There was the selection of Rahm Emanuel as White House chief of staff, of Timothy F. Geithner as treasury secretary.

“I had a lot of expectations, which were probably set by the media, that Obama was going to be able to do all of this stuff,” said Kathryn Babcock, 69, a retiree from Green Valley, Ariz. She had supported Clinton in the 2008 primary, then switched to Obama, and wept with joy when he was elected. “Then he brought in all of those Wall Street types, just like everybody else,” she said. “That was my first clue that this was going to be business as usual.”

According to many liberal activists, Obama’s centrist tendencies stopped him from seizing a moment. They said so at the time. In December 2009, Rolling Stone reporter Matt Taibbi accused the Obama administration of hiring “the very people who caused the crisis in the first place.” During the Affordable Care Act debate, many on the left insisted that the mandate-based compromise was not only worse but also less electorally viable than an expansion of Medicare or a “public option” to cover more people.

“I think the response early in the recession was probably more muted and quiet and moderate than it should have been,” said Rep. Raúl M. Grijalva (D-Ariz.), co-chairman of the Congressional Progressive Caucus. “Everybody was too afraid.”

The centrist Democrats called them crazy and self-destructive, a “professional left” — in the words of Obama’s first spokesman — with no grasp of strategy. The professional left had an answer to that. The centrists could not explain why a party that came out of the 2008 election with 59 senators in its caucus and 257 House seats now has just 46 and 188. If that was not inevitable, then a major political moment had been missed. Whose fault was that?

“How would the world be different today if, when the economic crisis had hit, Joe Stiglitz had been secretary of the treasury and Simon Johnson and Robert Reich had been the key economic advisers?” Warren asked in her Netroots Nation speech Friday morning, referring to three popular figures on the left. “Think about it.”