San Francisco — The surreal moment at the Green Party’s recent presidential debate in San Francisco came just after it ended, when candidate Roseanne Barr — yes, that Roseanne Barr — got campaign advice from punk pioneer and previous Green Oval Office candidate Jello Biafra.

He urged Barr to “use your humor.”

Barr, as famous for her groundbreaking 1980s sitcom “Roseanne” as for the tabloids’ documentation of her plastic surgery history, has been playing it straight — at least lately. But she is carrying a serious message for the party faithful searching for a way to grow.

So in deference to the Green’s anti-death penalty stance, the wealthy TV comedian and Hawaii macadamia nut farmer said she would stop joking about sending billionaires “to the guillotine.” And she’s played down an earlier shtick where she said she was simultaneously running to be prime minister of Israel. Or how Willie Nelson turned down her invitation to be her vice president because “he’s starting his own party.”

Perhaps more shocking is that in her blunt manner, Barr is addressing the Nader in the room for liberals who may agree with the Greens’ liberal platform but fear that voting for the party’s candidates could elect Republican Mitt Romney. It’s the lingering residue of the 2000 presidential race, when many Democrats believe that Green Party-nominee Ralph Nader’s 2.7 percent of the vote delivered the presidency to Republican George W. Bush.

Barr is telling conflicted liberals that if they’re going to vote for President Obama, at least register as a Green Party member because the exodus will shock the Democrats.

“For those people,” Barr told an audience Saturday night that didn’t quite fill the 480-seat Victoria Theatre in the Mission District, “if they could just leave the Democratic Party and register as Greens, they could still vote for Obama but it would be sending the Democratic Party itself a message it needs to hear.”

The audience cheered.

While that’s not the official stance of the Green Party, among those agreeing was Barr’s opponent, Jill Stein, a Harvard-trained physician and environmental health expert who has won the vast majority of Green delegates committed thus far and all of the primaries.

“A lot of people have been afraid to stand up as Greens” because of “a fear campaign that has been drummed into the American people” since 2000, Stein said. “What has come of this politics of fear? The politics of fear has brought us everything we were afraid of. Silence is not an effective political strategy.”

A dozen years after Nader put Greens on the national map, the party is struggling with how to grow. There is a lower percentage of Green Party members in California than in 2000 — still less than 1 percent of the state’s registered voters. It’s barely above that in uber-liberal San Francisco, Alameda and Marin counties.

While 133 Greens hold elective office nationwide, the highest-ranking party member in the United States is Richmond Mayor Gayle McLaughlin.

As Biafra, a Green activist who now is more of a spoken-word artist, asked Saturday, “Should there be a Green Party candidate for president at all, when the only momentum this party has is for local offices, where we win from time to time?”

There will be three Green presidential candidates on California’s June 5 primary ballot (San Diego County air quality inspector Kent Mesplay did not attend Saturday’s event), and Stein and Barr acknowledge that they have virtually no differences over policy. Barr spent most of the debate nodding and applauding as Stein enunciated in Clintonian detail a platform that could appeal to the most liberal Democrats.

They want single-payer health care; an end to involvement in foreign wars; a forgiving of all student loan debt; an end to the Keystone pipeline plan and a ban on hydraulic fracturing for oil, or fracking; heavy federal investment in a program to create 25 million green jobs; and the legalization and taxing of marijuana.

Barr, who told party leaders she would “barnstorm American living rooms” in her Green Party questionnaire, is seen as a way for the party to appeal to working-class voters.

“Greens have been known to be a little bit wonky,” said national party spokesman Scott McLarty. “Candidates will hand a voter a sheet with their entire policy positions on it.”

Barr moved several people in Saturday’s audience when she teared up talking about her modest roots.

There is little mention that Barr, who spends most of her time on a $1.7 million, 46-acre macadamia nut farm in Hawaii, is among the wealthiest of Americans. She’s done little in-person “barnstorming” outside of a near-constant stream of Twitter postings. She told The Chronicle that airplane travel makes her sick.

Despite her Twitter zeal, the policy positions on her website are skeletal.