With crushing housing costs in California showing no sign of easing off, a measure that would give cities far greater leeway to impose rent control is shaping up to be one of the hardest-fought initiatives on the November ballot.

The California Democratic Party voted over the weekend to endorse Proposition 10, which would repeal a law called the Costa-Hawkins Act that barred cities from capping rents on housing built after the measure took effect in 1995. That date is earlier in cities that already had rent control before the law’s passage. For example, rent control is limited in San Francisco to buildings built before 1979. In Oakland, rent can’t be capped on buildings built after 1983.

The initiative would also allow cities to limit price increases on rentals when they become vacant.

Fifteen cities in the state now have rent control, and as housing prices have shot up in recent years, more cities have considered limiting what landlords can charge.

Five Bay Area cities had rent-cap measures on the 2016 ballot, two of which were approved, in Mountain View and Richmond. Last year, voters in Santa Rosa and Pacifica rejected rent-control initiatives.

At its meeting Sunday in Oakland, 95 percent of the California Democratic Party’s executive board voted to back Prop. 10. That means the party endorsement will be on slate cards, direct mail and email blasts that reach as many as 2 million California Democrats.

“Securing the Democratic Party endorsement is huge,“ said Joe Trippi, the campaign’s lead strategist and longtime political adviser to Gov. Jerry Brown.

The ballot measure is supported by some of the state’s largest and deep-pocketed public employee unions, including the California Teachers Association, the California Nurses Association and the California branch of the Service Employees International Union.

“It gives space in our cities to our nurses, our janitors, the people working behind the counter at restaurants,” said Damien Goodmon, the Prop. 10 campaign director.

Opponents are lining up against Prop 10, too. On Friday, the California Association of Realtors announced it was giving $750,000 to oppose the measure, and Orange County developer Michael Hayde donated $3.7 million.

Developers and real estate agents found an ally in a group that could help them cut into progressive support for the measure, when the California NAACP announced last week that it opposed Prop. 10.

“It will make affordable rental housing even more scarce than it is today, widening the gap between our state’s haves and have-nots,” California NAACP President Alice Huffman said in a statement. “We need to increase the availability of affordable housing targeted to those most in need — but this initiative is the wrong approach that will only make the problem worse.”

On Monday, the opposition forces were joined by the Los Angeles/Orange Counties Building and Construction Trades Council, which represents more than 100,000 workers.

“We fight every day for working men and women and believe we need to protect Californians from escalating housing costs,” said Ron Miller, the union’s executive secretary. “However, our state cannot remedy its housing crisis without building new housing, and this initiative would halt construction in our communities, making our housing crisis worse.”

Prop. 10 opponents point to a 2016 report from the state’s nonpartisan Legislative Analyst’s Office, which concluded that expanding rent control was unlikely to increase the state’s housing supply “and, in fact, likely would discourage new construction.”

Goodmon says that assertion fails the eyeball test.

“Look at all the cranes in the sky in San Francisco. Look at the cranes in the sky in Los Angeles,” Goodmon said. “Those are hot real estate markets in cities that have rent control.”

Deep dives and late takes

California Republicans in races that could determine who controls the House went easy on President Trump for his soft approach toward Russian political interference.

Democrats in those races aren’t rushing to make the future of abortion rights an issue in the fight over confirming President Trump’s Supreme Court nominee.

Rep. Maxine Waters kept up her verbal war with President Trump, saying he “doesn’t know a darn thing” about NATO and can’t spell his wife’s name.

California’s laws about gun rights, greenhouse gases and sanctuary policies could come under scrutiny by a U.S. Supreme Court with another conservative justice.

There’s a reason San Francisco hasn’t opened a safe injection clinic for drug users — the possibility the Trump administration would prosecute city officials.

Advocates of an initiative that would divide California into three states made their case to the state Supreme Court for keeping the measure on the ballot.

California Democratic Party leaders delivered a stunning rebuke to Dianne Feinstein, endorsing election opponent Kevin de León over the state’s senior senator.

San Francisco Mayor London Breed named her replacement on the Board of Supervisors — her former legislative aide, Vallie Brown.

Republican John Cox wants to debate Democrat Gavin Newsom five times before the governor’s election. Newsom says once is more like it.

A San Francisco supervisorial candidates debate and appearance by a 2020 Democratic presidential hopeful highlight the Bay Area political calendar.

The Political Punch newsletter publishes every Tuesday and Thursday between noon and 3 p.m. It is produced by the staff of the San Francisco Chronicle and edited by politics editor Trapper Byrne. Email: tbyrne@sfchronicle.com