By Sharon Bernstein

SACRAMENTO Calif. (Reuters) - Californians head to the polls on Tuesday for a primary election likely to highlight divisions in the state's dominant Democratic party, as incumbent labor-backed candidates fight reformers positioning themselves to take on unions in several races.

The election to choose candidates for governor, secretary of state and numerous legislative and congressional offices could be the kickoff to a long and politically bloody election season for Democrats, as the state's open primary system allows the top two vote-getters, regardless of party, to square off against each other in November, and in many cases both will be Democrats.

"It’s going to be like scorpions in a bottle," said political analyst David Mark, editor of the Palo Alto based website Politix.

In Los Angeles, 18 candidates were certified to run for the seat being vacated by retiring Congressman Henry Waxman, including former City Council member and mayoral candidate Wendy Greuel, State Senator Ted Lieu and New Age inspirational author Marianne Williamson.

The two who receive the most votes Tuesday will then square off in a contest expected to get ugly quickly, Mark said.

Several races involve candidates who have been allied with the state's powerful labor unions running against fellow Democrats who are not.

In the contest for Superintendent of Education, incumbent Tom Torlakson has strong financial backing from teachers unions, while former charter school executive Marshall Tuck is calling for changes abhorrent to labor, including a lessening of seniority protections for teachers.

Silicon Valley Congressman Mike Honda has pushed for higher minimum wages and has strong support among labor unions. Opponent Ro Khanna, a lawyer and former U.S. Commerce Department official, has support from powerful tech leaders, including Facebook executive and "Lean In" author Sheryl Sandberg.

"It’s the sort of old school labor establishment vs. tech start-up clash I think we’ll see more of in coming years," said Mark.

In the race for governor, there is little doubt that the top vote-getter will be incumbent Jerry Brown, a Democrat who has steered the state on a moderate course and has high approval ratings.

Fissures in that race are more likely to show among Republicans, as state Assemblyman Tim Donnelly, an anti-illegal immigration activist with support from the party's conservative base, fights for a spot on November's ballot against Neel Kashkari, a moderate who worked for Presidents George W. Bush and Barack Obama.

(Reporting by Sharon Bernstein; Editing by Jeremy Laurence)

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