Still in the throes of primary season, the 16th Assembly District race has already made its mark as one of the most heated and highest-spending contests in the state this year.

Four candidates are vying for the seat being vacated by Assemblywoman Joan Buchanan, who is being forced out by term limits. They are Danville Mayor Newell Arnerich, attorney Catharine Baker, Orinda Vice Mayor Steve Glazer and Dublin Mayor Tim Sbranti. All but Baker are Democrats.

The district extends from east of the Berkeley hills, through Orinda and Walnut Creek, to the Tri-Valley. With the Democratic vote being split three ways and Baker likely to be popular with conservative voters, who often turn out strongly in primary elections, she may well survive the primary. But winning in this left-leaning district in the Nov. 4 general election may prove more difficult.

Thus, a battle royale has emerged between Glazer and Sbranti, the two Democratic front-runners with strong bases of moneyed support and on different sides of the pro-labor and pro-business divide. The high-stakes nature of the race has made it the leader in independent campaign expenditures in the state. As of Thursday, unions have spent about $1.3 million to back Sbranti, and Glazer's campaign has been bolstered by about $1.4 million from real-estate interests, charter school advocates and the state Chamber of Commerce.

"This is going to be the highest spending race in California -- it's such a big fight," Glazer said. And that, he said, is because "it's viewed as a bellwether race on whether or not candidates that hold centrist points of view can regain control of the Legislature."

Glazer describes himself as "fiscally conservative Democrat who has the courage and independence to stand up to these powerful interests." Pro-union forces, he said, "will say and do anything to stop me from being elected."

Glazer made a name for himself earlier this year as one of the few high-profile Democratic candidates willing to call for a ban on BART transit worker strikes. That placed him in direct conflict with powerful union interests and with Sbranti, a longtime teacher and labor activist.

Sbranti dismisses the BART strike ban as "political fodder" that detracts from real issues.

"I think the reality is when you actually talk to real voters, although (the strike) was frustrating ... there are far more pressing issues we have to deal with, such as maintenance issues, derailments and a lack of parking at BART stations," Sbranti said. "Those are things that affect people on a daily basis."

Arnerich isn't backed by special interest group money, and he bills himself as the only independent candidate in this race. He said Glazer's crusade against BART strikes came off as "a just a little disingenuous."

"You are grabbing at straws to make a name for yourself," Arnerich said. "And is this such an essential service that we want to take that right (to strike) away? I don't think you should be stomping on people's backs to make yourself look good."

Baker said she also strongly opposes BART strikes and that she is "the only candidate in this race that actually supported real legislation to put an end to the BART strike" -- a Republican-led bill that ultimately didn't get far in the Legislature.

The Glazer and Sbranti camps have waged war with waves of commercials and mailers accusing each other of being the Democratic candidate more deeply in the pocket of special interests.

Sbranti's supporters have blasted Glazer for being a longtime political strategist and paid political adviser in 2012 for JobsPAC, a group operated by the state's Chamber of Commerce. Big Tobacco companies were leading contributors to the PAC at the time, but Glazer has denied ever working for or taking direct contributions from tobacco companies. (Glazer's camp has attacked Sbranti for having the backing of unions, plus some big companies of his own, such as Chevron and PG&E.)

Glazer's camp has tried to paint Sbranti as so indebted to unions that he's anti-business, but Sbranti said that during his tenure as Dublin mayor, the city has had a strong track record of adopting business- and development-friendly policies.

Beyond those hot-button issues, both Baker and Glazer have listed pension reform as one of their top three priorities, while Arnerich proudly said Danville, where he's a five-term mayor, has no unfunded pension liability. Sbranti doesn't list it as a top priority. Baker strongly emphasizes the need to rein in the state's debt, more so than the other candidates.

All of the candidates list as priorities water management, improving educational opportunities and enhancing business and job prospects. They all oppose making high-speed rail a top priority over other, more pressing transportation issues, and they oppose Gov. Jerry Brown's proposal to build tunnels under the Delta to ship water to Southern California.

Aside from the more visible disagreements over BART strikes and union power, "I'm not seeing huge wedge issues where the candidates are that far apart," said Steve Woolpert, dean of liberal arts and a politics professor at Saint Mary's College in Moraga. "It may come down to endorsements and how well-funded the candidates are and how well they can promote their name recognition."

Contact Joyce Tsai at 925-847-2123. Follow her at Twitter.com/joycetsainews.

Newell Arnerich
Age: 61
Career or job: President and CEO of architectural urban design firm
Education: B.A. and M.A., UC Berkeley

Catharine Baker
Age: 43
Career or job: Small-business attorney
Education: B.A., University of Chicago; JD, UC Berkeley

Steve Glazer
Age: 56
Career or job: President, Glazer & Associates, a strategic consulting company.
Education: B.A. in business/political science, San Diego State

Tim Sbranti
Age: 39
Career or job: Dublin High School teacher; energy manager for Dublin Unified School District
Education: B.A. in government and minor in history and communications, Sacramento State