“He’s arguing with bold leadership we’re going to do it — even though he knows you need Trump’s waivers to do it,” Mr. Villaraigosa said. “They are not going to give us the waivers for that. Think about this: You ready to double your taxes? Nobody is going to do that. Everyone is for it until they realize — whoa. The cost. ”

Mr. Chiang said he, too, supported a state health care system. “I just think Gavin Newsom should be forthcoming on how they are going to pay for it,” he said.

Mr. Newsom dismissed his critics as “defeatist Democrats.”

“It’s a jaw-dropping thing: to say you support something but it can’t be done,” Mr. Newsom said in an interview a few blocks from his office in Sacramento. “Who the hell is interested in ‘it can’t be done’? We wouldn’t have marriage equality if I had listened to ‘it can’t be done.’”

Asked what his plan was to pay for it, Mr. Newsom responded: “The one that works is the one I support. I’m not an ideologue.”

Still, the biggest question may well be whether anyone will be able to replace Mr. Brown as California’s counterpoint to Mr. Trump — and whether the state’s stature as a national leader in this fight will diminish when Mr. Brown retires to his ranch next January.

“California is usually the leading edge,” said Ms. Eastin, one of the Democrats seeking to replace him. “It’s the sixth-largest economy in the world. I think California will get attention.”