This was supposed to be the year Republicans shook free of the Tea Party. Tired of alienating voters with radical candidates, went the theory, the GOP would “tone down the crazy” and run moderates.

California didn't get the memo. Polls say Tim Donnelly, a libertarian firebrand who has vowed to “annihilate” the enemies of freedom, is likely to be the party's candidate for governor.

The state assembly member from Twin Peaks – a remote, rural community – has roused grassroots conservatives with broadsides against taxes, illegal immigration and government regulation.

“We have tremendous momentum. When you become a champion of the people they'll follow you to the ends of the earth,” he told the Guardian during a campaign stop this week.

America's biggest state was hungry for his promise of smaller government, he said. “The Democratic party has been hijacked by Marxist progressives. Americans believe that government is the biggest threat to our future.”

California and the US needed to secure the border and deter illegal immigrants, added Donnelly, 47. “I don't blame them for coming here when we're giving them all kinds of free stuff. The very first thing we should do is stop rewarding illegal behaviour, stop incentivising it.”

Such rhetoric has powered Donnelly past Neel Kashkari, a better-funded and centrist rival who languishes in the polls. But it has alarmed some GOP activists, who fear the impact on efforts to rebrand the party.

Latinos, women and young people, in particular, are likely to recoil should Donnelly clinch the nomination, said Lionel Sosa, a Texas-based GOP media guru who advised Ronald Reagan, both Bushes and John McCain.

“We're making it very difficult to be likeable,” he said. “We have to remember that if people don't like us they won't vote for us.”

Few doubt that Governor Jerry Brown, a Democratic incumbent with 59% approval ratings and a $20m (£12m) campaign war chest, will squash whichever Republican runs against him in November.

“Brown would have to commit a major felony with a minor farm animal on TV to lose,” said Jack Pitney, a political scientist at Claremont McKenna college. The concern for the GOP, said Pitney, is that a Donnelly candidacy would tar Republicans running for other state offices – and the party's image beyond California.

A former Minuteman who led militia-style border patrols, the Golden State's GOP frontrunner favours Arizona-style laws against illegal immigration, which he has linked to gang violence, and opposes driving licences and public financial aid for undocumented people.

He has voted against bills on minimum wage and transgender rights, wants to abolish the Air Resources Board which regulates air quality, and calls state welfare programmes a form of slavery.

Since running for the gubernatorial nomination he has focused on crumbling infrastructure, shoddy schools and the flight of businesses to lower tax states.

“I love California,” he says, “and I hate what Jerry Brown has done to it.”

Donnelly is an easy target for liberals. Stephen Colbert skewered him on The Colbert Report in 2007, when he was a fence-building border activist, and John Oliver, then with The Daily Show, mocked him in 2011, after he was elected to the state assembly.



A poll earlier this month, however, gave Donnelly 17% of likely votes in the June primary. That was far behind Brown, on 57%, but way ahead of his GOP rival Kashkari, on 2%, suggesting a Brown vs Donnelly runoff in November.

Donnelly said he had more latino support than any other Republican in the state, despite his opposition to a general amnesty for undocumented people. “If we undermine the rule of law we will lose this republic,” he said. “We will get the same anarchy and corruption that all these people are fleeing.”

The California Republican National Hispanic Assembly is split over whom to endorse.

“Some of our members worry about his impact on latino voters but others are very impressed by his stance on other issues,” said the group's president, Errol Valladares, of Donnelly. “Personally I'm backing Neel Kashkari, he's the fresh face we need and will stand a better chance against Brown.”

Donnelly's rhetoric on immigration could mobilise the Democratic base, said Pitney. “Brown could use him as a bogeyman, say he's anti-hispanic and motivate hispanics to turn out and hurt other Republicans running for office.”

That scenario would abort GOP efforts to rebuild in California, where it has foundered since Arnold Schwarzenegger's stint as governor, and resonate beyond the state, he said.

“I expect Democrats nationwide to be mentioning Tim Donnelly a lot.”

Republican leaders have talked of expunging extremism, perceived or real, since Barack Obama turned Mitt Romney's anti-immigrant stance into a stick in the 2012 presidential race. Calls to reoccupy the centre ground swelled last November after Tea Party-backed candidates lost in Virginia and the moderate Chris Christie triumphed in New Jersey.

“Tone down the crazy,” conservative pundits counselled.

But in California the grassroots call the shots, making the Tea Party a resilient force. For Donnelly, his surge in the polls shows he is in tune with constituents fed up with what they see as dysfunctional, bloated government. He brushed off his prickly relations with the state's GOP establishment, a rump subject to Democratic dominance in Sacramento.

Republican grandees once dismissed another California radical who went on to become governor and president, he pointed out.

“Ronald Reagan figured out how to unite them. I believe the GOP will get behind me after I get the primary,” he said.

Donnelly bristled at mention of strategists like Karl Rove who wish to isolate the Tea Party. “The only thing matters to me is if you win. The real embarrassment is that they get paid even when they lose.”

Democratic California governor Jerry Brown is a strong favourite to be re-elected. Photograph: Justin Sullivan/Getty Images Photograph: Justin Sullivan/Getty Images

Originally from Georgia and one of 14 children, Donnelly obtained a degree in English from the University of California and set up a small business in Twin Peaks, where he lives with his wife Rowena, who is of Philippine descent, and their five children.

Few took him seriously when he first declared his ambition to be governor. He was best known for trying to board a plane with a loaded handgun in 2012, prompting a fine and three years of probation. Splitting from his campaign manager and then his legislative chief of staff did not help his shoestring campaign.

The Sacramento Bee has questioned his ability to last until June. “Severely underfunded and with little name recognition outside his district,” it said, “his paid media consists almost exclusively of web videos.”

Yet Donnelly remains the GOP frontrunner with a formidable lead over Kashkari and the endorsement of the conservative California Republican Assembly. “We've built an army [of volunteers] that's just about to canvass the streets of California,” he said.

With latinos becoming a plurality in California – a recent landmark – the state's Republicans risked prolonged exile in the political wilderness, said Sosa, the consultant. Governor Pete Wilson alienated latinos in 1994 by backing proposition 187, an anti-immigrant measure, he said, and the party was still paying the price.

“Latinos don't forget. Pete Wilson helped turn California into a Democratic state. People who take a hard stand on immigration are really saying we don't welcome hard-working people here. So many Republicans are making it hard for latinos to like Republicans.”