On Thursday afternoon, Vermont Senator Bernie Sanders took to the stage in a lush park in Springfield, Ore., and did something unusual. He departed from his usual stump speech and began talking about the future of the Democratic Party.

To reform a broken electoral system, the party needed to embrace an array of changes, he said. Elections should be publicly funded. Citizens should be automatically registered to vote when they turn 18. Primary contests should always be open to independent voters.

"Our job is not just to revitalize the Democratic Party," Mr. Sanders told his supporters. "Our job is to revitalize American democracy."

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Absent a political miracle or catastrophe, Mr. Sanders will not be the Democratic nominee for president. After a string of defeats in recent primaries, including in four of the five states that voted this week, Mr. Sanders finds himself well behind former secretary of state Hillary Clinton in the race for the nomination.

However, his impact on the Democratic Party – and the country's broader political alignment – is only just beginning. It may not be much consolation for his most fervent fans, but Mr. Sanders could turn out to be one of the most consequential losers ever in American politics.

A self-described socialist, Mr. Sanders has turned out to be a far more vigorous opponent than Ms. Clinton expected. He has built a tremendous following among millennial voters and a fundraising machine powered by small donors that is broader than anything assembled by any previous candidate, including President Barack Obama. Along the way, Mr. Sanders has exposed fault lines within the Democratic Party. While such rifts are not as explosive as those dividing Republicans, they reveal a strong desire for change among some Democrats.

Nowhere is that hunger deeper than among millennials. Earlier this week, the Institute of Politics at Harvard University released its biannual survey of voters under the age of 30. Its results were remarkable: Over the past year, their overall preference for a Democrat in the White House doubled. Their support for government action to curb poverty and climate change jumped, as did the popularity of the idea that health care is a basic human right.

Mr. Sanders has helped "crystallize their thoughts in terms of the role of government and their priorities," said John Della Volpe, the polling director at the institute. "He has had more of an impact than any other losing candidate that I can recall in a generation or two in terms of shaping the dialogue and ideology not just of the party, but of a generation."

For Mr. Sanders's army of young workers and volunteers, his campaign is a formative experience, which could, in some cases, shape their careers. "There is a future president and there are a ton of future members of Congress where the first thing they ever did in politics was feel the Bern," said Joe Trippi, a Democratic political consultant.

At the Democratic National Convention in Philadelphia in July, Mr. Sanders and his delegates will attempt to get their positions codified in the party's official policy platform. If that doesn't work, Mr. Sanders will likely have enough delegates to submit a "minority report" – a shadow version of the official document, which can serve as a rallying point for the progressive wing of the party.

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Mr. Sanders is also expected to use his fundraising network and popularity to benefit candidates elsewhere on the ballot. Indeed, he has already begun to do so, endorsing three women running for Congress in the fall – all of them progressive-minded, outsider Democrats like he is.

Some of Mr. Sanders's supporters are organizing a "People's Summit" in Chicago in June after the last primary votes are cast, an initiative independent of the campaign. "This is that moment where groups come together and say, 'What do we want to do?'" said Charles Lenchner, co-founder of People for Bernie, a grassroots outfit participating in the summit. "It's different from the mould of waiting for Bernie to decide what he wants to do."

It's unclear how Mr. Sanders himself intends to take his movement forward. In theory, he could run as an independent candidate, although observers say that's unlikely.

More probable, experts say, is that he will help to unify the party behind Ms. Clinton while trying to push its platform to the left. He could also use the assets of his thoroughly modern campaign – online organizing tools, fundraising from small donors, grassroots enthusiasm – to create something new. Back in 2008, Mr. Obama assembled something similar, but he went on to win, said Mr. Trippi, the Democratic consultant. Mr. Sanders "gets to invent what you do with all of this when you lose."

For Mr. Sanders's supporters, it's clear that the road does not end at the Democratic convention in July or even with the presidential election in November. Carla Bellamy, a 44-year-old professor at a public university in New York, has been a registered Democrat her entire life but had never volunteered for a campaign until Mr. Sanders came along.

Prof. Bellamy said that the campaign has led her to question whether broader change is best achieved within the Democratic Party or via some kind of alternative, like a multiparty system. But in the short run, she's proud of what Mr. Sanders has accomplished.

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"I don't think we can have an election any more where people don't ask hard questions about super PACs and money and who you're really working for," she said. "That's really important progress."