Big money from labor unions, real estate agents, tech executives — and even one candidate’s mother — is pouring into an open state Senate race on the Peninsula ahead of a fiercely contested March primary.

A crowded field is vying to replace Sen. Jerry Hill, D-San Mateo, who is termed out after eight years representing the 13th Senate District, a wealthy suburban stretch from Brisbane to Sunnyvale.

With more than half of voters registered Democratic and only 15% signed up as Republicans, a Democrat is almost certain to win the seat in November. But only two candidates can advance from the primary scuffle.

That has raised the stakes for the five Democrats in the race, several of whom are putting hundreds of thousands of dollars of their own money behind their efforts. If they can advance to the runoff, they may just have a clear path to victory.

Republican Alexander Glew and Libertarian John Webster are also on the ballot, though neither has reported raising much money.

A look at the leading candidates:

Josh Becker, 50, of Menlo Park has raised the most cash — more than $950,000 — and is endorsed by Hill and Gov. Gavin Newsom. He has received significant contributions from people in the tech industry, including Facebook executive Sheryl Sandberg. LinkedIn founder Reid Hoffman put $500,000 into an independent committee supporting Becker.

Becker said his background in social entrepreneurship would make him a “voice for innovation” in government. He started a biotech company, co-founded a venture capital firm focused on socially responsible businesses and created the Full Circle Fund, which provides grants and leadership training to Bay Area nonprofits.

His major priority is to be a leader on “progressive climate legislation.” Becker worked on environmental and energy policy in Washington, D.C., after college and started an early investment fund for clean energy businesses. He said it’s a strong foundation for integrating California’s transportation sector into its green energy grid and improving storage capacity so the state has clean electricity sources available 24 hours a day.

He wants to improve economic opportunity by better coordinating the schedules and fares of the Bay Area’s public transit systems and investing more in affordable housing. One idea he has floated is requiring big tech companies to fund a unit of housing with each job they create.

Shelly Masur, 55, a city councilwoman in Redwood City, says her experience in public education spurred her to run. She was on the local school board for a decade and lobbied the Legislature against budget cuts during the recession. Until recently, she was CEO of a public education policy nonprofit.

“I’ve seen it from both sides,” Masur said.

One of her top priorities is moving California from the bottom 10 states in per-pupil spending for K-12 schools to the top 10. She supports the likely November initiative that would change how property taxes are calculated for commercial properties, sending billions of dollars in new revenue to local governments and schools.

Masur has a master’s degree in public health and spent her early career defending abortion clinics. Those experiences motivate her to work on expanding health care access, she said, “so people don’t have to choose between eating dinner and going to the doctor.”

Politically powerful interests in both of those areas — teachers unions and the California Medical Association — are funding an independent expenditure campaign that has spent more than $170,000 in support of Masur. She has raised more than $460,000, with her largest donations mainly coming from organized labor.

Masur is also the only candidate in the race who endorsed SB50, San Francisco Democratic Sen. Scott Wiener’s controversial plan to spur denser housing around public transit and in wealthy suburbs. Masur said the measure, which failed last month, would have provided a framework to address California’s housing shortage and climate goals together. She worked with Wiener on amendments giving cities more time to develop alternative local plans.

“He did really bring the issue of housing to the forefront,” Masur said. “And love or hate SB50, everyone has been talking about the issue of housing.”

Mike Brownrigg, 58, served for more than a decade as a U.S. diplomat in Syria, Hong Kong, China and Washington, D.C., which he said gave him the experience to build bridges between people with competing interests and get things done at the state Capitol.

Now a city councilman in Burlingame who has worked in venture capital and impact investing, Brownrigg’s agenda for Sacramento is “affordability in the morning and climate in the afternoon.”

Although he sees a role for the state to set housing targets for cities and ensure they meet those goals, he objected to SB50 because it reflected a view that “the main problem is local officials not approving things.”

Brownrigg said the bottleneck is in financing and that California should focus on boosting funding. He proposes to help nonprofits buy buildings where rent is relatively low, essentially turning them into affordable housing.

“Cities should have a crack” at reaching their housing goals, he said, “and if they can’t get it done, then we can have a harder conversation.”

Brownrigg plans to pursue legislation transitioning California to a carbon-free energy system by 2030 and reducing single-use plastics. He supports the commercial property-tax initiative and other efforts to increase school funding.

He has raised nearly $470,000, and his campaign has been boosted by $350,000 of his own money. Last month, his mother put $460,000 into an independent committee to elect him. He argued it was less problematic than interest groups backing other candidates, because she would expect nothing in return except “that I come to visit more often.”

Sally Lieber, 58, of Mountain View, calls herself the most liberal candidate in the race — a “minimum wage, pro-environment, pro-choice, Medicare for All kind of progressive Democrat.”

She was in the state Assembly from 2002 to 2008, which she says taught her tenacity: It took three tries to pass her biggest accomplishment, a bill raising the state minimum wage to $8 an hour, then the highest in the country.

Lieber wants to address income inequality by expanding the earned income tax credit for the working poor, getting the state to the top five in per-pupil spending and reducing the cost of Caltrain so more people can afford to take it. She also said the state must focus on building low-income housing, an area when it has “dramatically underperformed.”

Since leaving the Legislature, Lieber has worked on environmental advocacy in the Bay Area.

Her campaign is largely funded by $200,000 from her own pocket, which she said has always been her approach to campaigns and gave her “a real sense of freedom in the Capitol” during her Assembly tenure.

Annie Oliva, 60, a real estate agent and city councilwoman in Millbrae, said a close connection to California’s homelessness crisis was her call to action to run for Senate. Her son struggles with addiction and mental health issues and has been homeless, off and on, for years.

“So when I see a homeless person, I know it could be any one of us,” she said. “These people are people.”

Oliva’s plan on homelessness involves declaring a state of emergency, expanding the use of conservatorships and increasing funding for prevention services.

It also includes building more housing, which she wants to concentrate around public transit. Oliva said the state must upgrade its transit systems so more people will use them. But she does not support SB50, which she said would destroy single-family neighborhoods.

Oliva has raised more than $400,000 and has backing from an independent committee funded by the California Association of Realtors, which has spent more than $475,000 on her behalf.

Alexei Koseff is a San Francisco Chronicle staff writer. Email: alexei.koseff@sfchronicle.com Twitter: @akoseff