SACRAMENTO — California property taxes are one area that U.S. presidents have no control over, but that hasn’t stopped candidates for the Democratic nomination from lining up behind a prospective ballot measure to overhaul Proposition 13, the state’s landmark tax-cutting law.

The audience for the presidential hopefuls’ message is not so much ordinary Californians — most of whom are not yet paying close attention to a 2020 measure that would boost property taxes for large businesses — as it is organized labor, a key Democratic constituency.

The state’s powerful public employee unions have made the initiative an election-year priority. If passed, it would unleash billions of dollars for schools and local governments that employ union workers.

And for Democratic presidential candidates jockeying for labor’s backing, showing support for key goals such as the Prop. 13 overhaul is crucial. A union endorsement could bring money and volunteers for their campaigns ahead of California’s March primary.

Former Vice President Joe Biden offered his support last week, becoming the eighth presidential candidate to endorse the proposal to raise taxes for large commercial properties while keeping Prop. 13’s limits in place for homeowners. All the leading contenders, including Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, are now on board with the measure, whose backers are collecting signatures to qualify it for the November 2020 ballot.

“It’s about labor,” said Andrew Acosta, a Democratic political consultant who is not involved with the ballot measure or any of the 2020 candidates’ campaigns. “That’s the game inside the Democratic primary right now.”

Other California union issues, such as a bill that reclassifies gig workers and many additional independent contractors as employees, as well as a labor dispute at UCLA, have already drawn the attention of presidential hopefuls this year.

Getting them to endorse the property tax initiative is a big push for the Service Employees International Union California and the California Teachers Association, which represent hundreds of thousands of members who work for cities and schools that would benefit from the extra funding. Both are likely to spend millions of dollars on a campaign. Although neither group makes endorsements in the presidential primary, they are part of national unions that might. Some of their leaders and members are part of those discussions.

“That’s been the No. 1 ask for any candidate that comes to California,” said Alex Stack, a spokesman for Schools and Communities First, the coalition of labor, education, philanthropic and social justice groups behind the initiative.

Backing Prop. 13 overhaul Democratic presidential candidates who have endorsed California’s split-roll initiative, which would raise property taxes for large owners of commercial property. Joe Biden, former vice president Cory Booker, New Jersey senator Pete Buttigieg, South Bend, Ind., mayor Julián Castro, former secretary of Housing and Urban Development Kamala Harris, California senator Bernie Sanders, Vermont senator Elizabeth Warren, Massachusetts senator Beto O’Rourke, former Texas congressman who has since quit the race

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Proponents began circulating a new version of their proposal last month to split the property tax roll. It would maintain Prop. 13 limits for residential, small business and agricultural property while eliminating those protections for business and industrial buildings and commercial land worth more than $3 million.

Passed by voters in 1978, Prop. 13 capped state taxes for all types of property at 1% of the purchase price, with annual increases of no more than 2%. Critics have long complained that California is losing out on billions of dollars in revenue each year from valuable business properties that have never changed hands and thus never had their taxes reassessed.

The initiative would ask voters to adopt a system known as “split-roll,” under which the annual tax bill for large commercial and industrial buildings and land would be based on their market value. The windfall, estimated at between $7.5 billion and $12 billion a year, would be split between cities, counties and special districts, which would get about 60%, and schools and community colleges, which would receive the rest.

Supporters will have to collect nearly a million signatures by April 21 to qualify their proposal for the ballot.

Sanders was the first presidential candidate to back the idea, at a July conference hosted by United Teachers Los Angeles. He told the crowd that “the time is long, long overdue” to revisit “tax breaks and loopholes to billionaires or real estate developers when 500,000 people are sleeping out on the streets tonight and when our kids are not getting the education they deserve.”

Warren offered her support on Twitter in September, and last month, she filmed a video for Schools and Communities First in which she criticized Prop. 13 for “gutting resources for the most vulnerable” to “pad the pockets of the wealthy and well-connected.”

Since the beginning of October, more endorsements have come from presidential candidates, including Biden; Sens. Kamala Harris of California and Cory Booker of New Jersey; Mayor Pete Buttigieg of South Bend, Ind.; former Secretary of Housing and Urban Development Secretary Julián Castro; and former Rep. Beto O’Rourke of Texas, who later dropped out of the race. Several specifically mentioned they were standing with SEIU California and the California Teachers Association, each of which has put half a million dollars into the signature-gathering effort.

Stack, the spokesman for the initiative campaign, said the endorsements “show how important this really is.” He said supporters did not expect much further participation from the candidates, though they hope to bring more attention to their proposal at the California Democratic Party convention this weekend in Long Beach, which many of the leading contenders are planning to attend.

A year out from the election, prospects for the measure look shaky. A September poll by the Public Policy Institute of California found that 47% of likely voters surveyed favored the split-roll idea, while 45% opposed it.

Acosta, the Democratic political consultant, said he doubted any endorsement would have a major impact on those numbers, other than perhaps backing by Gov. Gavin Newsom, who has a more direct connection to state policy. Newsom has expressed a desire to reach a compromise for broader changes to the state tax structure that would avoid a contentious ballot fight.

Noting the middling poll numbers for the proposed initiative, John Kabateck, state director of the National Federation of Independent Business in California, said the “proponents of the $12.5 billion a year split-roll property tax hike are grasping at straws” by seeking the support of presidential candidates. His group is involved with the opposition campaign.

“Every small business — even those that rent their properties — will be forced to pay,” Kabateck said in a statement. “This will ultimately result in higher rents for mom-and-pop businesses and higher prices for the goods and services we buy every day.”

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