Donnelly, with 10 percent support among likely voters, outpolls his closest GOP competitors by 8 percentage points, according to a Public Policy Institute of California poll released Wednesday.
Neel Kashkari, a former U.S. Treasury Department official and the best-funded Republican in the race, was supported by 2 percent of likely voters, as was Laguna Hills Mayor Andrew Blount.
All Republicans trail Brown by an enormous margin. The third-term Democrat is supported by 47 percent of likely voters, while 36 percent of likely voters remain undecided, according to the poll.
Brown is widely expected to win re-election, but the race for second place in June will determine which Republican represents the GOP in the state's highest-profile race in the fall.
Donnelly, a tea party favorite and former member of the anti-illegal immigration Minuteman Project, leads Kashkari, a more moderate candidate, among all voter groups, including Republicans, independent voters and Latinos, according to the poll.
Kashkari has time to make up ground. In addition to the large percentage of voters who remain undecided, only 37 percent of likely voters say they are following news about the candidates closely at this early stage in the race.
The poll comes at the end of an up-and-down month for Donnelly. He was cheered by activists at the California Republican Party's convention outside San Francisco, but his campaign manager, Jennifer Kerns, left the operation, and Donnelly continued to struggle to raise money.
Donnelly reported Monday that he has less than $11,000 in cash on hand, with unpaid bills of $149,068. Kashkari, meanwhile, has banked more than $900,000, while Brown has nearly $20 million on hand.
Addressing a Republican group in Danville on Tuesday, Donnelly acknowledged needing money but said he said he could run an effective, low-cost campaign.
The poll comes as the state continues to grapple with drought, and it found voters today more likely than a year ago to favor an $11.1 billion water bond currently scheduled for the November ballot.
The poll found 50 percent of likely voters support the measure, up from 42 percent a year ago. Support increases when voters are asked what they would think about a less expensive bond, and lawmakers are discussing a variety of proposals to replace the measure currently on the ballot.
PHOTO: Republican gubernatorial candidate Tim Donnelly speaks with reporters at the California Republican Party's biannual convention in Burlingame on March 15, 2014. The Sacramento Bee/David Siders