There are 27 gubernatorial candidates, and winners in the primary will advance to November's election. So far, polls show Lt. Gov. Gavin Newsom leading the pack, and there's a fight for second place between former Los Angeles Mayor Antonio Villaraigosa and Republican businessman John Cox.
Newsom cast his vote about 10:30 a.m. He has said he would prefer to face a Republican as his opponent.
Villaraigosa campaigned furiously in the final days to try to close the gap and appears to have the best shot at being one of the top two. He also voted on Tuesday in Los Angeles.
Meanwhile, recent polls showed a surge by Cox, who was endorsed by President Donald Trump. Republican registration has fallen for years throughout the state and the party has not won a statewide contest since Arnold Schwarzenegger was re-elected governor in 2006.
For more information on all candidates for California governor, click here.
RELATED: California Primary Elections 2018: Where and when to vote
California's U.S. Senate primary will determine whether 26-year incumbent Dianne Feinstein faces a fellow Democrat who characterizes her as too deferential to the president or someone from a field of largely unknown Republicans.
Feinstein, 84, is seeking her fifth full Senate term. She points to her seniority on key committees and track record of fighting for gun control and protecting the environment as evidence she deserves another term.
Her most likely challenger is Kevin de Leon , the former state Senate leader from Los Angeles who authored the "sanctuary state" bill to protect immigrants living in the country illegally. It has drawn repeated scorn from President Donald Trump.
De Leon, 51, also promoted clean energy initiatives during his tenure leading the Senate and argues he's more representative of California's values.
Thirty other candidates are running, including eight more Democrats, 11 Republicans, two third-party candidates and nine candidates not affiliated with a party.
For more information on all senate candidates, click here.
Democrats must wrestle at least 23 seats from Republicans to gain control of the House of Representatives. Some of those key races are happening in California.
Republican Congressman Ed Royce is retiring after 26 years in Congress. He represents the 39th District, which includes Los Angeles, Orange and San Bernardino counties.
Another retiring Republican is Congressman Darrell Issa. His 49th District covers the southern coastal area of Orange County and parts of San Diego County.
Orange County Congressman Dana Rohrabacher is being targeted by multiple Democrats as he runs for re-election. However, the Democrats may split up their vote so much he may be able to hold onto his seat.
Only the top two vote-getters will be on the November ballot, regardless of party.
Secretary of State's guide to the candidates:
CALIFORNIA PRIMARY PROPOSITIONS
Californians are also voting on five proposed statewide ballot measures.
Proposition 68: The measure would let the state borrow $4 billion through general obligation bonds for parks, water infrastructure and conservation projects. It includes more than $1.2 billion to build and improve state and local parks. It would also authorize $550 million to guard communities against flooding and $200 million to help preserve the shrinking Salton Sea.
Supporters include conservation groups and water agencies.
Proposition 69: The measure would amend the state constitution to require money raised from a new diesel tax and from vehicle fees to be spent on transportation projects. Lawmakers put this proposition on the ballot when they passed a gas tax hike last year.
Supporters include groups representing construction companies and workers, who could benefit as the state spends more to fix roads and other transportation projects.
Republicans want to put a separate initiative on the November ballot that would repeal the gas tax increase. They argue that even if Proposition 69 passes, creative lawyers will find ways to divert the money elsewhere.
Proposition 70: This proposition would require the Legislature to vote in 2024 on how to allocate revenue from the cap-and-trade program, which generates billions of dollars annually by requiring polluters to buy permits to release greenhouse gases.
Existing state law requires a quarter of the money to be spent on the state's high-speed rail project, which Republicans generally oppose. Proceeds also go to affordable housing and transit projects. The 2024 vote would be a one-time reset of the spending plan and require support from two-thirds of lawmakers to pass.
The measure could give Republicans a greater say in how the money is spent and was part of a deal last year to extend cap-and-trade to 2030.
Proposition 71: The measure would change the effective date for propositions from the day after the election to five days after election results are certified. This amendment to the state constitution pushes the effective start date for voter-enacted policies back about six weeks. Backers say it would ensure all votes are counted before new policies are enacted.
Proposition 72: The proposition would amend the state constitution to let Californians install rain-capture devices without increasing their property taxes even though they enhance property values. Supporters say the tax break would encourage people to install the water-saving devices, which can be helpful in a state prone to drought.
Secretary of State's guide to the propositions:
L.A. County Registrar Dean Logan said there are more than 21,000 community volunteers working at more than 4,000 polling sites. Polls will close at 8 p.m.
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The Associated Press contributed to this report.