"The Sanders campaign is absolutely destroying us."

Those are the words of California Green Party spokesman Mike Feinstein, who, in response to an inquiry from Mother Jones on Friday, visited the website of the California Secretary of State. He discovered, to his consternation, that his party has lost 30 percent of its members in the months since Sanders launched his presidential campaign. "I am apoplectically mad right now," Feinstein says. "I am so disgusted with this."

"They intentionally went after our voters because they are low-lying fruit on the issues," he adds, citing mailers the Sanders campaign sent to Green Party members.

The steep drop in Green registration underscores how Sanders has energized California's far-left electorate.

The party's steep decline in registration—from nearly 110,000 voters in early 2015 to 78,000 now—represents a tiny fraction of California's 18 million registered voters. Yet it underscores how the Sanders campaign has made deep inroads into California's liberal electorate, tapping voters who may have never before considered voting for a Democrat.

California's other major leftist third party, the Peace and Freedom Party, has also seen a significant drop in registration since last year, losing about 7,000 voters, or 9 percent of its members.

"Most of the members of our Central Committee, and probably other registrants, like Bernie," says Debra Rieger, the Peace and Freedom Party's state chair. Two of the party's three presidential candidates are themselves socialists, and their policy positions aren't appreciably different from Sanders'. "We think it's great that Bernie has opened to door to talking about socialism, free education for everyone, open healthcare—all these things we've been advocating for years."

With Sanders and Hillary Clinton locked in a statistical dead heat in California, at least according to the polls, a Sanders victory here may hinge on his ability to mobilize even more of these ultra-left voters. But consolidating that fractious group is no easy task, even for a democratic socialist who regularly sounds the themes of the Occupy movement.

Consider that Occupy Oakland has not promoted any events featuring Bernie Sanders—not even his Monday visit to Oakland's Frank Ogawa Plaza, the site of the group's original occupation. Instead, last week, Occupy Oakland urged its 47,000 Twitter followers to attend a Friday afternoon Berkeley rally hosted by Green Party presidential candidate Jill Stein.

"These folks don't hate the Democratic Party as much as they hate what it has become."

Stein attributes this support to her rejection of the Democratic Party and uncompromising stance on issues such as Palestinian rights. "Our campaign has liberty that Bernie Sanders does not because we are not on the leash of a corporate party sponsored by war profiteers and Wall Street banks," she told me. "Bernie has been restrained."

The Sanders campaign also faces a practical limitation: Democratic Party rules in California allow only registered Democrats, independents, and decline-to-state voters to participate in the party's primary. Greens and Peace and Freedom partiers who support Sanders would have had to re-register by May 23 to cast a ballot for him.

Stein bristles at the notion that her campaign could be a spoiler in the Clinton-Sanders race: "This is basically a propaganda campaign, a disinformation campaign," she says. "The reality is that the lesser of two evils is not a solution."

Sanders' supporters here have spent months responding to such arguments. Tom Gallagher, a former chairman of the Democratic Socialists of America, has represented the Sanders campaign in two debates with left-wing political parties in the Bay Area, with another planned for Saturday at a Berkeley pub. He wrote a book several years ago arguing that leftists should occupy the Democratic Party.

"To me, this is the most important campaign in 40 years," says Gallagher, a former Ralph Nader supporter who is now a Democratic Party delegate representing San Francisco. "If we want [socialism] in play in this country we've gotta be in the presidential election—that's when people think about big ideas."

"There is a real option now," says a Ralph Nader supporter turned Democratic delegate. "Argue over November later."

Many activists agree. The web page of Occupy San Francisco has promoted Sanders events, and Bay Area for Bernie has signed up several former Occupy people as volunteers. Among them is Sierra Madre, the moderator of its Facebook page, who was at the 2011 Occupy Oakland protest where police seriously wounded a protester with a teargas canister. "These folks don't hate the Democratic Party as much as they hate what it has become," Madre says. "They see that they have the chance to change it to make it more populist, more working class, and there are seizing that opportunity by voting for Bernie."

Sanders has said he will use his delegates to push for changes at the Democratic National Convention. Among other things, he wants it to move to fully open primaries in every state, which would enable members of third parties to cast ballots for Democrats without re-registering.

Gallagher argues that voting for Sanders in the primary isn't necessarily a vote for the two-party system. "There is a real option now," he says. "Argue over November later."

Reiger of the Peace and Freedom Party expects that her missing members will come back after the general election—and possibly bring along some new ones. "The Democrats will never allow [Sanders] to be president," she says, "but we will be very happy to welcome those people into our ranks."