California Needs More Affordable Homes. This Union Stands in the Way.

California legislators proposed more than a half dozen major bills last year to address the state’s affordable-housing crisis, which researchers say is one of the worst in the nation. None of them passed.

Most died or were withdrawn, according to people involved in the processes, in large part because of campaigns waged against them by the state’s powerful construction-workers union.

California’s State Building and Construction Trades Council, which represents 450,000 ironworkers, pipe fitters and other skilled laborers, has blocked numerous bills it says don’t guarantee enough work for its members. It contributes tens of millions of dollars to political candidates and campaigns, engages in aggressive lobbying, and pays for advertisements that portray opponents as lackeys of greedy developers. Legislative insiders say the success of the union known widely as “the Trades” is one of the main reasons Sacramento politicians have struggled to pass bills streamlining construction approval and easing zoning restrictions. Researchers say those steps are urgently needed to address skyrocketing real-estate prices and rents, as well as homelessness.

“They’re a gatekeeper for any significant legislation moving through Sacramento” on housing, said Ben Metcalf, managing director at the Terner Center for Housing Innovation at UC Berkeley and former head of the state’s Housing and Community Development agency.

Despite the state’s robust economy, about 7.1 million of California’s 40 million residents live in poverty when factoring in housing costs, according to the California Budget & Policy Center. A 2018 report by the state’s Department of Housing and Community Development found California has a shortage of 1.5 million affordable rental homes for low-income families.

At the heart of the dispute is the Trades’ insistence that proposals to incentivize home building require certain numbers of construction workers be graduates of apprenticeship programs that are mostly union-run. Easing restrictions, they argue, makes a developer’s land more valuable and that wealth should be shared through labor protections and higher wages. They also say that the standards, known as “skilled and trained,” help combat minimum-wage abuses in the construction industry.

“You cannot build affordable housing and address poverty by driving construction workers and their families into poverty,” said Robbie Hunter, president of the State Building and Construction Trades Council.

Builders say apprenticeship requirements drive up the already sky-high expense of affordable-housing construction in a state where it can cost as much as $700,000 a unit to build in dense, urban areas such as San Francisco. They also argue that the union-backed provisions could slow or halt construction of affordable homes in lower-income rural and inland areas where there isn’t enough available union labor.

Housing advocates want the ability to forgo union labor on projects where a developer doesn’t get any bids that fulfill the skilled and trained requirements, or if those bids are significantly higher than the lowest alternative.

In a state dominated by Democrats, deference to organized labor isn’t novel. The California Teachers Association, the state’s largest teachers union, wielded immense power in the debate over when to reopen schools during the Covid-19 pandemic. Nurses’ unions hold sway over healthcare legislation. In addition, local governments and neighborhood groups have opposed many housing bills.

The Trades are among Sacramento’s most prolific donors. Since 2015, the State Building Trades and its affiliated local unions have given more than $90 million to state candidates and campaigns, according to an analysis by California Target Book, a nonpartisan state election guide. About $16.6 million came from the main statewide umbrella organization led by Mr. Hunter, who said affiliated locals make their own campaign decisions.

Gov. Gavin Newsom, who clashed with the Trades in 2019 over bills to expand the number of housing projects requiring union wages, has attempted to broker deals between unions and industry in the past. This year he is seeking support from organized labor as he fends off a likely recall election. The Democrat’s office declined to comment.

Last May, days before a key legislative deadline, the Trades objected to about a half dozen housing bills, some of which they had previously said they wouldn’t oppose, according to legislators, staff members and advocates. They included proposals to make it easier for religious institutions and nonprofit hospitals to build affordable housing on lots they own, loosen restrictions on cities that want to allow more multiunit zoning, and streamline the process of converting vacant hotels and motels into permanent housing for low-income residents.

The latter bill’s author, Susan Talamantes Eggman, said at a hearing that she didn’t know why the union spoke out against the bill. The then-assemblywoman had already included its labor standards, she said, but asked that projects that were 100% affordable be exempted.

“We get to vote on what we think is good public policy even when sometimes that means saying no to our friends,” the Democrat said, her voice breaking. “When I drive my car now with my 11-year-old daughter and I see people laying on the street, do I tell her to turn her head? Do I tell her the State of California can’t help?” The union said it was opposed because workers on the projects would be “paid barely above the minimum wage to toil on dangerous construction sites.”

The bill passed out of the housing committee but died later in another committee. Ms. Eggman revived it this year after being elected to the state Senate, but included provisions requiring skilled and trained workers for all projects. In a letter expressing its support, the union called the bill “consensus-driven housing policy as it should be.”

Housing advocates say they have unsuccessfully pushed for a compromise with the Trades that would allow legislation to speed construction to pass this year if it includes more union jobs.

“Until we come to a resolution, it’s going to make housing policy very hard in California,” said State Senate Housing Committee Chairman Scott Wiener.

But conflicts have continued in the past few months.

Assemblyman Richard Bloom withdrew a bill last year to allow affordable housing on some commercial lots after the Trades opposed it. After he reintroduced it this year, the union ran a full-page ad in a local newspaper depicting the Democrat as a Godzilla-like figure towering over a city skyline, asserting that he was “in the pocket of developers” and intent on “destroying Santa Monica neighborhoods.”

“It’s clear what the intention is here, and that is, ‘Don’t mess with us,’ ” said Mr. Bloom, a Democrat.

Mr. Hunter said he plans to continue with what he sees as effective advocacy for his members. “We’re lifting the working person from the bottom up and we do have strong opinions,” he said.

Write to Christine Mai-Duc at christine.maiduc@wsj.com