OAKLAND — Public-sector unions are facing an existential threat and need stronger support from elected officials, candidate for governor Gavin Newsom argued at a firefighter union’s block party Saturday, three days before the primary election.

“Organized labor is under assault in this country,” Newsom, standing on the back of a shiny red fire truck, told a group of about 100 firefighters and other attendees in downtown Oakland. “We’ve got your back.”

The Democratic lieutenant governor has won the backing of California’s largest firefighters, teachers, nurses and state employees unions, which have bankrolled independent groups supporting his campaign.

Many of those groups are now bracing for the impact of Janus v. AFSCME, a U.S. Supreme Court case that could be handed down as soon as next week. Legal observers expect the Court, which has a conservative majority, to strike down rules requiring public employees pay union dues to fund collective bargaining efforts. That would hamstring unions’ ability to fight for their workers’ rights, Newsom and other supporters say, although proponents of the lawsuit argue it would prevent employees from having to fund activities they disagree with.

If the justices make union payments voluntary, Newsom vowed to support a campaign urging public employees to stick with their unions. “We’re going to have to make the case for the membership at each and every local,” he told reporters, as his 2-year-old son Dutch sat in the front seat of the fire truck nearby. “We have to make a case anew why unions matter.”

“Democrats have, frankly, taken unions for granted,” he added.

That rhetoric was music to the ears of Zac Unger, the vice president of International Association of Fire Fighters Local 55, which represents Oakland and Alameda County firefighters. If Newsom needs help in the general election, “we’ll put boots on the ground to get him over the finish line,” Unger said. The group’s statewide affiliate, the California Professional Firefighters, has already given $250,000 to an independent political committee backing Newsom.

Elsewhere in the state Saturday, former Los Angeles Mayor Antonio Villaraigosa and State Treasurer John Chiang, both Democrats, rallied voters in Los Angeles and Orange counties, while Republican businessman John Cox attended church services in San Bernardino County. Villaraigosa will be in San Jose on Sunday, meeting voters with Mayor Sam Liccardo, and Newsom will be back in San Francisco on Monday night after another swing through SoCal.

Several voters at the firefighters’ event, an annual chili cook-off that was also organized by Assemblyman Rob Bonta, D-Oakland, said they were unhappy with the torrent of negative ads flying in all directions in the governor’s race.

“It’s confusing — with all the mudslinging on TV, you don’t know what to believe,” said Kelly Smith, an insurance agent in Danville, who hadn’t decided which candidate to vote for.

After snapping selfies and chatting about the Warriors with voters, Newsom and his family sampled several of the competing chilis in the cook-off, including one featuring brisket that had been smoked for 12 hours. But he declined to be a judge in the event, saying that would violate “rule number one” for elected officials — “because you’re guaranteed to anger” somebody.