Click here if you are having trouble viewing the slideshow on a mobile device.

He is pulling ahead in the polls, has built an army of volunteers and just became the first non-billionaire 2020 presidential candidate to drop a television ad in California.

With ballots about to hit voters’ mailboxes, Bernie Sanders’ supporters across the Golden State are feeling the surge.

And what timing: The primary season kicks off Monday in Iowa, where Sanders also holds a narrow lead in recent polls. And with California’s Super Tuesday primary on March 3, the Vermont senator appears to be breaking away from Sen. Elizabeth Warren of Massachusetts and former Vice President Joe Biden after being caught in a three-way statistical tie in the state for months.

In what once seemed unthinkable, the 78-year-old democratic socialist who suffered a heart attack on the campaign trail in October has emerged as one of the front-runners for the Democratic nomination and a showdown with President Donald Trump.

Anything can happen in the month before California’s Super Tuesday primary, which will be deeply shaped by the results in the first four early states. But after years of organizing for Sanders — who narrowly lost California to Hillary Clinton in the 2016 primary — his biggest Golden State fans are fired up.

“In 2016, I really liked what he had to say, but I didn’t think he could realistically win,” said Sarah Norr, a 39-year-old drum teacher who organized dozens of Sanders supporters at her home in Oakland’s Fruitvale neighborhood on Saturday to knock on doors for him. “A lot has changed in four years. Now, we can really do this.”

California is central to Sanders’ strategy. His campaign has built a massive organization up and down the state, opening more offices than all of his rivals combined. The team is focusing on the millions of voters who will receive their ballots in the mail starting Monday.

And last week, Sanders became the first non-billionaire candidate to launch a substantial ad buy in California’s pricey TV markets, dropping $2.5 million on spots promoting himself in English and Spanish in California and Texas, another Super Tuesday state. He’s still getting far outspent on the airwaves by former New York City Mayor Michael Bloomberg and former San Francisco hedge fund chief Tom Steyer, who are self-funding their campaigns with their massive fortunes.

Sanders is able to afford the ads after raising $34.6 million in donations between October and December, more than any of his rivals, according to campaign finance data released Friday.

Still, Sanders’ rise has set off alarm bells among Democratic establishment figures, both in California and nationally, who have long written him off and worry that nominating him could doom the party’s chances of taking out Trump in November.

Supporters of moderate candidates are urging Californians to vote with their heads instead of their hearts and back a more centrist contender who they say would be a better fit for the heartland swing states that will likely determine the general election.

“It is really critically important for all of us as Californians to recognize that we live in a bubble and the rest of the country doesn’t see the world exactly as we do,” said San Jose Mayor Sam Liccardo, Bloomberg’s California campaign co-chair, when asked about Sanders’ surge last week. “I am concerned about whether or not the Democratic Party can nominate a candidate who can defeat Donald Trump in Ohio and Michigan and Wisconsin and Pennsylvania and all those places where we lost in 2016.”

Even candidates like Steyer, who says he respects many of Sanders’ ideas, are suggesting they’d be better positioned than the senator to stand up to Trump’s general election broadsides.

“Trump is running on the economy,” Steyer pointed out in a recent interview. “He’s going to claim that Bernie is a socialist who will destroy the economy. Bernie is a socialist, self-avowed, that’s not an accusation or a cut-down. He’s a socialist, and he’s proud of it.

“Trump is gonna say that about every Democrat,” added Steyer, who built a fortune of $1.6 billion through his San Francisco hedge fund. “He can’t, obviously, say that about me.”

But Sanders’ fans argue that he can inspire voters who are less likely to turn out in November, while appealing to working-class Americans who leaned toward Trump during the last election.

In 2016, “the biggest advantage Trump had was a very mobilized, passionate base,” said Rep. Ro Khanna, D-Santa Clara, Sanders’ national campaign co-chair. “Senator Sanders is the person on our side who can most match that energy. He has the grassroots army that it’s going to take to defeat Trump.”

Head-to-head polls between the Democratic candidates and Trump show Sanders defeating the president with a margin only slightly smaller than Biden, who continues to lead the Democratic field nationally. But Sanders’ opponents point out that the Vermont senator has faced less scrutiny on the national stage, and would take on a flurry of attacks if he becomes the nominee.

As Sanders jumped in polls around the country, that has already started. In Iowa, he’s getting hit by negative ads from two independent expenditure groups, including one that explicitly mentions his recent heart attack. The incoming fire will only pick up if Sanders does well in Iowa and New Hampshire, the first two states.

One of Sanders’ biggest strengths in California is his support among Latinos, who make up about 35 percent of the state’s adult population. The latest poll released by the UC Berkeley Institute of Governmental Studies last week found him with 38 percent support among likely Latino Democratic primary voters, 20 points ahead of Biden, who was in second with the group.

Part of that is Sanders’ appeal among young voters. And his campaign has also emphasized his immigrant history: Sanders’ father “didn’t know how to speak English and had almost no money” when he came to the U.S. from Poland at age 17, the narrator says in Sanders’ new Spanish-language TV ad. “Bernie never forgot his family’s immigrant roots, and that’s why he’s always fought for us.”

Historically, Latino voters have gone to the polls at a far lower rate than other Californians — one reason the campaign hopes to boost turnout with heavy investments in grassroots organizing.

Sanders’ team says they’ve opened 20 offices and hired 90 staffers up and down the state. In addition to a large presence in Democratic strongholds in the Bay Area and Los Angeles, the campaign has opened five offices in the Inland Empire and three in the Central Valley — areas with large Latino populations that typically get far less attention from the party’s presidential contenders.

“We are usually the kind of area that if you see the candidates, the campaign may show up once or twice,” said Tisa Rodriguez, the chair of the Riverside County Democratic Party, who is supporting Sanders. “It is highly unusual to have that large a presence here.”

The Bloomberg and Steyer campaigns say they’re also in the process of building huge California organizations. Bloomberg’s campaign has 220 paid staffers in the state, according to a spokesman, by far the most of any presidential campaign. They have opened five offices so far, with plans for at least 17 by election day. Bloomberg, who is skipping the four early states, is scheduled to campaign in Sacramento, Fresno and Compton this week, his fourth visit to the state in just over two months as a candidate.

Steyer’s camp has six offices in California, including their national headquarters in San Francisco, and about 70 staffers focused on organizing here. They plan to open at least four additional offices in February and grow the California-focused headcount to about 100 by election day, a spokeswoman said.

Warren’s campaign has offices open in Los Angeles, Oakland, and San Francisco, while Biden’s camp says they plan to open an office in the state soon. Other contenders stress they’re holding volunteer events to mobilize their supporters in California, even without a brick-and-mortar presence.

Sanders’ grassroots organization was out in force Saturday morning in Fruitvale, thanks to Bernie 2020 T-shirt-clad volunteers like Michelle Galecki, 24 and Adrián Arrivillaga, 26, a nonprofit worker and therapy student who live in Oakland.

Walking by squat houses and orange and lemon trees heavy with fruit, the young couple met Sanders supporters like Landon Williams, a 76-year-old retiree who said he liked the senator because he was “fed up” with establishment Democrats — and because he went to school with the actor Danny Glover, one of Sanders’ celebrity backers.

Other voters like Stephanie Hollier, 62, were more skeptical. “Bernie’s a little old,” she said, adding that the main thing she cared about was beating Trump.

Galecki pointed out that Nelson Mandela was only three years younger than Sanders when he was elected to lead South Africa. “I’m not from Africa, so that doesn’t mean anything to me,” Hollier responded, as her dog Pebbles strained to squeeze out the screen door.

Sonya Hope, 48, a medical worker who was visiting family from Tracy, said she was still making up her mind about which candidate to support. “Convince me,” she told the canvassers with a grin. They pitched her on the Green New Deal and Medicare-for-All, with Arrivillaga talking about his experience with therapy patients who don’t have insurance. By the end of the spiel, Hope was nodding along, and promised to consider Sanders.

“Bernie’s agenda isn’t radical — it’s what people in this country actually want,” Arrivillaga said as the duo prepared to head to another house. “If we can get that message out, we can really win this.”