Perhaps only in California can a politician who has championed single-payer health care, a “sanctuary state” bill and a 100 percent clean energy mandate be tarred for not being liberal enough.

But for some die-hard activists, Kevin de León’s history of taking money from corporations and his ties to the Democratic Party establishment make him a nonstarter in his insurgent bid to take on Sen. Dianne Feinstein.

Now a handful of long-shot challengers are running in the tiny sliver of space to de León’s left, complicating the State Senate leader’s attempts to consolidate liberal support and paint himself as the progressive alternative to Feinstein. Most notable is Alison Hartson, an activist and former high school teacher from Orange County who became a Democrat to vote for Sen. Bernie Sanders and has had surprising success racking up Sanders-style small-dollar contributions.

Hartson — who’s never run for office before and who’s received almost no mainstream media coverage of her campaign — received more money in small-dollar donations than Feinstein and De León combined: She took in $158,707 in unitemized contributions less than $200 over the last three months of 2017, compared to $111,524 for Feinstein and just $31,427 for De León.

Overall, de León raised $434,000 in that period, compared with $213,757 for Hartson, while Feinstein raised about $1 million from donors and political committees and gave her campaign a $5 million personal loan. Most of de León’s haul came from bigger donors, including architect Frank Gehry, tech investors Sam Altman and Reid Hoffman, philanthropist Laurene Powell Jobs, L.A. Dodgers president Stan Kasten and former Assembly Speaker Fabian Núñez.

With four months to go until the June 5 primary, de León has struggled to raise his poll numbers — the most recent poll found him at 17 percent compared to Feinstein’s 46 percent. Most public polls have ignored the other candidates so far.

De León is still a clear favorite over Hartson and other liberal hopefuls — especially after he was endorsed this month by two influential unions, the SEIU and the California Nurses Association. He has wider name recognition and far more prominence in political circles.

But the other candidates’ presence could be a hurdle for his campaign. If they’re invited to debates, for example, they would distract from de León’s attempts to go toe-to-toe with Feinstein. And if a major Republican does jump in — none has so far — the other liberals could peel away votes from de León and make it harder for him to get past the top-two primary.

“If you’re Feinstein, this is one time where you wouldn’t mind many of those Democrats in the race,” said David McCuan, a political science professor at Sonoma State University.

Bill Carrick, Feinstein’s longtime political strategist, certainly isn’t complaining. He said he was especially impressed with Hartson’s online fundraising.

“I think she’s going to be a presence in the race, definitely,” Carrick said. “She’s very clear on what she wants to say, and I think she’s going to have an appeal to some activists.”

When asked about the other rivals, de León spokesman Jonathan Underland turned the focus back on Feinstein. “Kevin, the other candidates in this race and all of our donors can agree on one unifying truth: Senator Feinstein is hopelessly out of touch with everyday Californians and her record of inaction and timidity is the embodiment of everything that’s wrong with Washington, D.C., today,” he said.

Hartson, 37, was born two years after Feinstein became San Francisco mayor. The sixth of nine children, she grew up in Orange County and was the first in her family to graduate college, at Cal State Long Beach. She taught English at a Garden Grove public high school for 10 years, launching a program to give struggling students more intensive help.

Watching public budget cuts made Hartson realize the importance of getting money out of politics, she said, and she became the director of WolfPAC, a group advocating against the Supreme Court’s Citizens United decision, which allows unlimited corporate spending on campaigns. She ran a campaign pushing California legislators to pass a resolution calling for a constitutional convention to override Citizens United.

She had tough words for Feinstein.

“Her stances on the issues are really stuck in this kind of 1980’s law-and-order style of governing,” Hartson said, citing the incumbent’s support for foreign interventions, domestic wiretapping and the Bush tax cuts. “She just very simply has not progressed with California, and it’s time for a new generation to lead.”

And while she supports many of de León’s policies — including his plan to require the state to use only sustainable energy by 2045 — she knocked him for raising money from corporations during past campaigns. “To be a good progressive you have to refuse to take that money,” Hartson said, calling him a career politician. (Corporations cannot give directly to campaigns for federal offices but can to campaigns for state offices in California.)

Hartson was recruited to run by Cenk Uygur, the liberal news host who launched The Young Turks, an online political show with an impassioned following on the left — and who briefly considered running against Feinstein himself. Hartson launched her campaign with an appearance on The Young Turks, and promotion by the show fueled her fundraising prowess.

Rep. Ro Khanna, D-Santa Clara, who is also backed by Uygur’s anti-establishment group, Justice Democrats, has endorsed de León against Feinstein. But while he considers de León the best candidate, “I’m really glad that Alison’s in the race,” Khanna said. “I think she’s bringing an energy from progressives I haven’t seen, and she’s not to be underestimated.”

Still, yet another far-left candidate, David Hildebrand, a Democratic socialist and union member in Sacramento, beat out de León and Hartson for the endorsement of one Sanders-associated Democratic club in Los Angeles after a debate last month.

Also in the race is Pat Harris, a defense lawyer from Los Angeles who’s represented police brutality victims and other civil rights cases. In one high-profile Santa Clara County trial in 2012, Harris successfully defended a man accused of beating up a priest who he said raped him as a child.

Harris, 59, who grew up in Arkansas and got his law degree at the University of Michigan, loaned his campaign more than $200,000 and raised another $100,000 over the past year, and says he’s willing to self-fund more. Now he’s traveling around the state in a bus that runs on solar power and ethanol, focusing on rural communities in the Central Valley and Inland Empire.

One issue with Harris’ campaign: If he won, California would face the confusion of having two Senator Harrises. “That,” he joked, “is one problem I’d love to have to deal with.”