SACRAMENTO  (AP) — A Democrat almost assuredly will win a state Senate seat in an east San Francisco Bay Area district where the party holds a 15-point registration edge. So why have unions poured $115,000 into a committee to support a Republican who dropped out of the race weeks ago?

Blame Democratic infighting.

Labor unions are trying to ensure that Steve Glazer, a Democrat seen as anti-union, doesn’t defeat two other Democrats who are considered pro-labor: Assemblywoman Susan Bonilla and former Assemblywoman Joan Buchanan.

The 7th District race, one of three state Senate races on the ballot Tuesday, is emblematic of the battle within the state Democratic Party between those like Buchanan and Bonilla who are viewed as supporting union ideals and expanded social welfare spending and others like Glazer and Gov. Jerry Brown who preach fiscal prudence.

Underscoring the high stakes is the $2.1 million pumped into the race by independent expenditure committees — campaign organizations set up by special interests.

Glazer, a longtime political adviser to Brown, is challenging the Democratic Party orthodoxy.

Among the tactics in play are mailers urging Republicans to vote for Michaela Hertle, a first-time GOP candidate who dropped out a week after entering the race but whose name remains on the ballot. Hertle endorsed Glazer, but fliers from the Asian American Small Business PAC to Republicans say “let’s elect one of our own.”

The PAC has mostly supported Democratic candidates of Asian descent; Hertle is neither.

Bill Wong, who runs the committee, said the group backs Hertle because she is the only small businessperson on the ballot. He said he has not spoken with the unions that have poured money into the PAC in recent weeks about why they did so.

Hertle said the mailers are disingenuous; she has recorded automated calls on Glazer’s behalf urging Republicans to ignore them. Under California’s open primary, registered voters can cast their ballot for any candidate, regardless of party.

“If the labor unions don’t like Steve Glazer, there is nothing about me that they would like,” she said in an interview. “So the fact that they are spending a lot of money to promote me under this guise, that’s just false principles on their part.”

After running Brown’s 2010 election campaign, Glazer earned the enmity of labor unions in 2012 by working for a Chamber of Commerce-funded committee to unseat incumbent Democrats in the Legislature. He then called for a ban on transit strikes as Bay Area Rapid Transit workers threatened a walkout. He also posted online some of the dozens of questionnaires sent to candidates by special interest groups in exchange for endorsements and financial support.

“It’s about the special interests controlling the Legislature. They do that through campaign expenditures and through secret questionnaires,” Glazer said.

Glazer’s donors include charter school proponents and Silicon Valley venture capitalists. Southern California businessman Bill Bloomfield Jr. has spent $550,000 backing Glazer, and the Chamber of Commerce-run JobsPAC has spent about $300,000 backing Glazer and opposing Bonilla and Buchanan.

On the other side, the California Nurses Association, Service Employees International Union and the State Building and Construction Trades Council of California and other unions have poured $260,000 into a separate committee opposing Glazer. The California Teachers Association spent nearly $150,000 on Buchanan’s behalf.

Democrat Terry Kremin is on the ballot but has not raised enough money to meet the minimum reporting threshold.

Bonilla, of Concord, said she tells constituents who have received attack mailers to “throw them in the trash and read your voter pamphlet.”

“I don’t approve of any of it and think that what the constituents and voters deserve is an honest, straightforward message about where I stand and how I have served them,” she said.

Buchanan, of Alamo, said she would prefer to eliminate independent expenditures. She rejected the idea that the race is part of a broader struggle over the direction of the party.

“This idea that there’s only one moderate candidate is just not accurate. I’ve had a reputation as being a moderate Democrat for decades now,” she said.

Tuesday’s election is a replay of a nasty Assembly race last year in which unions spent heavily to defeat Glazer, leading to a Republican victory in a district where Democrats held an 8-point registration edge.

Brown has not weighed in on Glazer’s bids for higher office, but his sister, former state Treasurer Kathleen Brown, endorsed his Senate campaign.

The race to replace Sen. Mark DeSaulnier is one of three on Tuesday’s ballot after former senators were elected to Congress. Assemblyman Don Wagner faces fellow Republicans John Moorlach and Naz Namazi in SD37 in Orange County, and former Sen. Sharon Runner, a Republican, is the only candidate in SD21.

In the East Bay race, the infighting will continue. The top two candidates will advance to a May 19 runoff.