SACRAMENTO — After months of anticipation and speculation, Gov. Jerry Brown on Thursday finally made it official: He’s running for an unprecedented fourth term as California’s chief executive.

Tweeting a photo of himself picking up the necessary paperwork at the Alameda County Registrar of Voters Office eight days before the filing deadline, Brown posted a letter on his campaign website touting his achievements over the last three years, maintaining that he had kept the campaign promises he made to voters four years ago.

Brown said he has helped pull California out of a multibillion-dollar budget hole, expanded the rights of illegal immigrants in the face of Washington gridlock over the issue and aided workers by creating a million new jobs and increasing the minimum wage.

“I said that I would work with both Democrats and Republicans, oil companies and environmentalists, unions and businesses, and I have,” Brown wrote. “I promised that there would be no new taxes unless the people voted for them, and you did.”

Brown will face two Republican challengers in June’s open primary — former U.S. Treasury official Neel Kashkari, who has staked his campaign on pledges to spur job creation and improve the public schools, and Assemblyman Tim Donnelly, R-Hesperia, a gun rights advocate whose candidacy is favored by the tea party. The top two candidates will face off in November.

Jack Pitney, a political expert at Claremont McKenna College, said he found Brown’s announcement “about as surprising as a Kardashian getting into trouble.”

“Brown enters the race as a heavy favorite to win in November,” he said, “but I wouldn’t say it’s a dead-in-the-bag certainty.”

Recent polls indicate that Brown, who served two terms as governor from 1976 to 1983, stands a good chance of winning the election in a landslide.

A poll conducted by the Public Policy Institute of California in January found that 53 percent of likely voters would pick Brown, while 17 percent favor Donnelly. Respondents were not asked for their opinions of Kashkari because he had not yet announced his candidacy when the poll was taken.

The poll also found that 60 percent of likely voters say they approve of the way Brown is handling his job, up from 49 percent in December. And the governor’s job performance won praise from more than 3 in 4 Democrats and a solid majority of independent voters, who often swing California elections.

Brown is also winning the fundraising war.

Last month, records showed Brown had already amassed a roughly $17 million campaign war chest from a broad coalition of supporters, including some businesses, like Chevron, that supported billionaire Meg Whitman, the 2010 Republican candidate for governor, when Brown ran against her.

Brown’s opponents have raised a fraction as much. Kashkari’s campaign has received a little more than $1 million in checks, while Donnelly’s camp has raised even less.

A spokeswoman for Donnelly could not be reached for comment on Thursday.

In a statement, Kashkari criticized Brown for “touting a status quo” that has been devastating for millions of families and communities across the state.

“Californians can’t afford another four years of Gov. Brown’s failed leadership,” Kashkari said. “It’s time for a new leader in Sacramento who will do the hard work of creating good jobs and giving every kid a quality education.”

Contact Jessica Calefati at 916-441-2101. Follow her at Twitter.com/calefati. Read the Political Blotter at IBABuzz.com/politics.