Donnelly, a gun-rights advocate, outspoken critic of illegal immigration and social conservative, said he is unfazed by a state electorate that leans far to his political left. He says his "guerrilla grass-roots" campaign will offer voters an alternative to the high taxes and what he calls government interference offered by Brown and his fellow Democrats.
"I can unite the divided majority that makes up California: people who work hard, who play by the rules and just want to be left alone by their government," Donnelly, 47, said in a telephone interview with The Associated Press.
Yet his candidacy, along with that of former lawmaker and Lt. Gov. Abel Maldonado, a moderate Republican, is unlikely to receive a warm welcome from the GOP establishment. After decades of sinking registration numbers, the party has sought to rebrand itself so it can appeal to a wider variety of voters, including Latinos.
Many party delegates had hoped to shift the party to the political center and keep the focus off the divisive issues that have been central to Donnelly's previous campaigns, including his history as a former Minuteman border enforcer.
Tenoch Flores, a spokesman for the California Democratic Party, said state Republicans are following congressional Republicans, who talked about rebranding the party after losing to President Barack Obama in 2012 but instead supported a partial federal government shutdown this fall.
"If anything, this just shows that Republicans are far from being able to move to the center and move to where Californians are," Flores said.
Mindful of the need to connect with a broader base of voters, Donnelly on Tuesday was flanked by a diverse group of supporters at the furniture factory in the Los Angeles County city of Baldwin Park where he announced his candidacy.
The official filing period to run for governor does not begin until Feb. 10.
The party's official position will be less relevant for any of the Republicans in 2014, the first year in which candidates for governor are subject to the state's top-two primary. In that system, the two highest vote-getters in June advance to the November general election.
Whoever emerges will face a daunting challenge in Brown, the 75-year-old governor who returned to office in 2011 after first serving from 1975 to 1983. Brown has yet to announce a bid for re-election, but there is little doubt that he will run.
After becoming the longest-serving governor earlier this month, Brown quipped last week that: "I'm only in the third year of my first of my second terms."
Brown has a 49 percent approval rating among likely voters, according to a September Public Policy Institute of California poll, and has nearly $16 million in his two campaign committees. He also successfully shepherded a budget through the state Legislature this year with little drama after voters approved billions of dollars in temporary sales and income taxes last year.
Donnelly, who represents a conservative Inland Empire district, has less than $40,000 combined in his two campaign accounts. Although his candidacy is likely to appeal to tea party sympathizers, Donnelly said he believes it will also resonate beyond the far right.
"I know that people have lost faith that you can win anything with grass roots, but I think we're living in a unique time in California, where there's so many people that feel like there's something wrong," Donnelly said.
He is the lone state lawmaker to support a proposed recall by gun owners' groups against five Democratic lawmakers who supported a variety of firearms bills this year. Donnelly is also known for being stopped last year at Ontario airport with a loaded gun in his briefcase. He was fined $2,215 for two misdemeanor charges and said he "made an honest, innocent mistake."
Donnelly and Maldonado could also face a challenge from a political and social moderate who has been courting donors: Neel Kashkari, the former assistant secretary of the U.S. Treasury who oversaw the federal bank bailout under both Presidents George W. Bush and Obama.
Kashkari, 40, has set up a website and hired consultants but has not yet decided whether to run, said his spokesman, Aaron McLear, although "he continues to travel the state to determine how he can help solve the problems facing Californians."
Maldonado, 46, the state's former lieutenant governor who ran unsuccessfully for Congress last year, initially launched a campaign challenging Brown's realignment law, which has shifted thousands of lower-level offenders out of state prison and into county supervision.
He has since shifted back to his more moderate image and announced his support for gay marriage. On Tuesday, he released a "day in the life" video of himself on the campaign trail.
Maldonado has raised $313,000 so far this year but spent much of it on a high-priced campaign team that he has since fired.