Presidential candidate Bernie Sanders greets people after speaking at a rally at the Anaheim Convention Center on Tuesday. (Matt McClain/The Washington Post)

— The deadline to register for the June 7 Democratic primary here was looming. As people trickled into a Bernie Sanders rally at Santa Monica High School, Josh Ehrhart was chasing the clock — and confronting Sanders supporters reluctant to become Democrats. Ehrhart offered registration forms and pens and assured them it would be harmless and temporary.

“I’m all about who the candidate is,” said Ehrhart, 32, who left the Green Party to vote for Sanders. “If Bernie stays in, obviously I’ll stay a Democrat. If he signs up to be Hillary’s vice president or something like that, I’ll stay a Democrat. If not, I’ll probably go back to the Green Party.”

Little about the 2016 primaries has been predictable, but one exception is Sanders’s antiseptic approach to the party whose nomination he is still trying to win. The longest-serving independent member of Congress has declined to officially become a Democrat while insisting the party must change.

He has also built an army of supporters who feel much as he does — that the primary process has been rigged against him and that Democrats care too much about the 1 percent. And even as front-runner Hillary Clinton expects to clinch the nomination on June 7, the squeamishness of these voters threatens her ability to unify the left. It may be her top challenge as she turns toward her ­general-election battle against Donald Trump.

Asked Monday why he wasn’t telling voters to become active Democrats, Sanders pronounced the question “esoteric.”

“That decision is obviously up to each individual voter,” Sanders said. “When you have a whole lot of folks in this country who are not happy with the Democratic Party, even less happy with the Republican Party, and for a variety of reasons many people are registering as independents or not affiliated, those people have got to be brought in to the Democratic Party.”

That kind of talk thrills Sanders supporters who want a political revolution from the outside. And it vexes those with Clinton, who know they must bring the left together if they are to defeat Trump in November. The dynamic is also frustrating to some liberal activists who see Sanders’s movement squandering a chance to effect a revolution from within the party.

“That’s one of the places this supposed movement falls short — lots of people who don’t want to be sullied from all the supposed corruption in the party but refusing to do the hard work of taking over a party from the inside,” said Markos Moulitsas, founder of the Daily Kos website and a key figure in the liberal Net-roots movement — and a voter in Berkeley.

Sanders and his supporters are reveling in an inside-outside game, trying to influence the party while building a force outside it. They have decried the “disenfranchisement” of independents who have been unable to vote in closed primaries, arguing that they would not just make the party more open but improve it.

By calling for more open primaries, Sanders is asking Democrats to consider whether a party can even win without allowing in people who have given up on ordinary politics. In polling conducted for the Washington Post and ABC News, both parties are less popular today than they were before the 2008 election. Shortly before they elected Barack Obama, 58 percent of voters said they viewed the Democratic Party favorably and 47 percent said the same of Republicans. Last week, the respective numbers were 48 percent and 36 percent.

At the grass-roots level in California, Sanders supporters operate with the knowledge that “Become a Democrat” is a flawed pitch. In a pre-deadline registration video, the loofah-haired comedian Reggie Watts told voters they could register as Democrats or “no party” on “whatever glowing rectangle you’re obsessed with.” Patti Davis, 66, ran a weekend phone bank to nudge non-Democrats to switch parties and found many willing to back Sanders if it didn’t mean a long-term commitment. (Those registered as “no party” can still vote in the primary with a special ballot.)

“A lot of people have a feeling that the corruption and the rigged primary system are horrible,” Davis said. “Each conversation’s a little different, but with the voters we were talking to, there was a strong sense that party politics isn’t working.”

Sanders’s massive California rallies are full of the skeptics, refuting the idea that independents are looking for a path between left and right. Steve Stokes, an activist who ran as an independent for Congress in 2014, shows up to as many as he can with fliers for his “Berniecrat” bid for U.S. Senate. Plenty of Sanders supporters bemoan the idea of identifying with a party.

“We want to be able to pick individuals, not parties,” Laurie Kasparian, 65, said at Sanders’s rally in Irvine.

“I wish you could vote as a citizen,” said Stephen Hughes, 51, who showed up at Sanders’s rally in east Los Angeles with registration forms. “I lived in Canada for two years, and I think the way they do it there is right, with a lot of choices.”

Cornel West, one of Sanders’s five appointments to the Democratic National Committee’s platform panel, has campaigned with likely Green Party presidential nominee Jill Stein. Black Lives Matter activist Shaun King and former labor secretary Robert Reich, both high-profile Sanders backers, wrote pieces this month suggesting that liberals look outside the Democratic Party once (and if) Donald Trump loses November’s election.

“If Hillary Clinton is elected, I urge you to turn Bernie’s campaign into a movement — even a third party — to influence elections at the state level in 2018 and the presidency in 2020,” Reich, who served under President Bill Clinton, wrote on Facebook this week.

To join or not to join has been an easier question in previous intraparty insurgencies. Jesse Jackson’s two presidential bids registered more black voters as Democrats; Howard Dean’s 2004 bid led to a successful four years atop a growing DNC.

The question of whether to take the Sanders movement elsewhere was debated as long ago as March in the influential socialist magazine Jacobin.

“I think the real challenge is turning an electoral movement into a non-electoral movement,” said Bhaskar Sunkara, the editor of Jacobin, “turning energy not only down-ballot . . . but turning a portion of Sanders supporters into social-movement activists broadly in the networks of the left.”

The Democratic Party’s establishment, which largely got behind Clinton early last year, has gone back and forth about Sanders’s efforts. This year’s contest, like 2008’s, has driven up voter interest and turnout. Both Clinton and Sanders are on track to win more votes than Al Gore or John F. Kerry did in winning their much-less-turbulent primaries. According to one measure, Democratic voter registration in California is up 218 percent from a comparable period in 2012; Republican registration is up 78 percent.

“How would I describe myself? Euphoric,” said John Burton, the chairman of the California Democratic Party. “I can’t remember the last time the numbers looked like that.”

Senate Minority Leader Harry M. Reid (D-Nev.) said to reporters this week: “I think we should just kind of lay off Bernie Sanders a little bit, okay? He’s done I think some really good things. The party has changed during his tenure here. And we’ll see what happens. I think Bernie’s a good man. He tries to do the right thing. And I think everything will work out well.”

Some Democrats worry after seeing similar movements burn then fade. Burton hopes the Sanders movement will avoid the mistakes of the effort that pushed Sen. Eugene McCarthy through the 1968 Democratic primaries.

“They got disappointed and stayed home, and we got Richard Nixon,” he said.

Tom Hayden, an early leader of Students for a Democratic Society who became a California legislator as a Democrat, said that he saw something “worse than 1968” in the Sanders supporters who promised to fight past the primary. He pointed to the furious response of some Sanders backers to Nevada’s Democratic convention, where a result that favored Hillary Clinton was described as fraud.

“I’m taking it for granted that they will do damage,” Hayden said. “Subconsciously, they’ll feel vindicated if Trump is elected president. They’ll take credit if Trump is defeated and cast blame if Trump wins. They’re already arguing that the primary was rigged, the platform’s going to be rigged, the ticket is going to be rigged, everything is going to be rigged, because they can’t step out of the rhetoric they’ve put out there.”

This week, after the Nevada anger subsided somewhat, Sanders made some moves that Democrats liked. On Sunday, he endorsed the Democratic primary challenger to Rep. Debbie Wasserman Schultz (D-Fla.) — hardly a “party unity” move but one that telegraphed that his supporters should stay involved with the party. On Tuesday, he backed eight Democrats running for state legislatures across the country. One of them, California’s Jane Kim, had even endorsed Clinton for president.

“Lots of us may be skeptical of Sanders himself, especially as he appears to be dead-set on finishing this primary with bitterness and recriminations,” Moulitsas said. “But if this is about harnessing that power to get more progressive Democrats elected at other levels of government? I’m ready to sign up.”

Mike DeBonis in Washington contributed to this report.