SACRAMENTO — With President Obama back from a trip to Alaska in which he portrayed the fight against climate change as an urgent international priority, California is showing how hard it can be — even in a state overwhelmingly controlled by Democrats — to get an ambitious carbon reduction bill passed.
The state has been at the forefront of global efforts to battle greenhouse gases, enacting mandates to force sharp reductions in emissions over the next 35 years. Its environmental record was applauded by Mr. Obama last week, and Pope Francis invited Gov. Jerry Brown to discuss the fight against global warming in the Vatican this summer.


But a centerpiece of California’s long-term campaign against emissions — legislation requiring a 50 percent reduction in petroleum use by Jan. 1, 2030 — has set off a fierce battle here, pitting not only a well-financed oil industry against environmentalists, but Democrat against Democrat. The bill easily passed the Senate, but it is faltering in the Assembly because of opposition by moderate Democrats, many representing economically suffering districts in central California. A vote is expected early this coming week.
           
The legislation faces an onslaught by the Western States Petroleum Association and other oil industry advocates that, in ads and mailings, assert that a 50 percent cut in petroleum use could result in gas rationing and a ban on minivans.
“This law will limit how often we can drive our own cars,” a narrator in one advertisement says urgently, an assertion the bill’s sponsors say is groundless. The oil industry has tagged the bill “the California Gas Restriction Act of 2015.”
A defeat would be a setback for Mr. Brown, who has made a battle against global warming a centerpiece of his final years in public life, and for environmentalists who have looked to California to lead the emissions fight at a time of strong skepticism about global warming in Washington. Mr. Obama urged California lawmakers to enact the bill in a recent speech in Las Vegas, signaling the importance he is attaching to the issue in his final years in office.
The environmental fight here comes on the eve of the United Nations climate change conference in Paris, set to take place in the fall. There, Mr. Brown and Kevin de León, the State Senate Democratic leader who led the fight for the bill in his chamber, are planning to outline for an international audience California’s campaign against greenhouse gases. On Wednesday, the Legislature passed and sent to Mr. Brown a measure requiring the state’s public pension funds to divest from coal companies.
“The rest of the world is watching very closely what is happening in California, and I think so far they see a success story,” Mr. de León said. “Our economy has grown — we are adding jobs, and we are reducing our carbon emissions. Therefore it is absolutely crucial that this measure passes, because it will be a big blow to the rest of the states and the whole world if it doesn’t.”
The legislation, Senate Bill 350, leaves it to the state’s Air Resources Board to determine how the 50 percent mandate would be met; it does not mention gas rationing or a ban on minivans. It also includes no penalties in case the mandate is missed. Opponents, in defending the warnings about rationing, noted that the bill is short on specifics on how the reduction would be achieved; they said they saw no other way the mandate could be met.
“I can’t figure out any other way to reach a 50 percent reduction in that frame without doing some pretty dramatic measures,” said Catherine Reheis-Boyd, the president of the Western States Petroleum Association. “If it isn’t gas rationing, what is it? I keep hearing what it isn’t.”
Mr. Brown said in an interview in his office here that the oil industry was using fear tactics to try to derail the effort before the Legislature adjourns on Friday, but that he was confident of eventual success.
“You’ve got the oil companies fighting Pope Francis,” he said. “Fighting the scientists of the world. Fighting the governor of California. They are engaged in literally a life-and-death struggle, and I have no doubt who is going to be the victor.”


He added: “It’s a shameless effort to maintain their revenue stream — regardless of what the impact is on everyone else. There is no rationing in the bill. Read it. None.”
The concerns have come not only from Republicans, but also from moderate Democrats who represent communities in central California. Many of these communities are struggling with high unemployment and slow economic growth.
“So much of our economy is driven by the use of petroleum,” said Assemblyman Henry T. Perea, a Democrat from the Central Valley and a leader of moderates in his house. “We don’t know what impacts S.B. 350 will have on it. We don’t know because we don’t know what the plan is. What does that look like? We haven’t heard that answer to that. And in the absence of information, you create your own.”
Kristin Olsen, the Assembly Republican leader, said her party was eager to find ways to curb harmful emissions. “My son has asthma — of course I want clean air,” she said.


But she questioned why California had to be a leader in an effort that she argued had such significant economic costs.
“We want to be leaders,” she said, “but not when there are no followers. And at some point we have to look at the fact that no one is following California’s lead. We are less than 1 percent of the world. At some point we should work on reasonable, cost-effective measures to reduce greenhouse gas emissions to improve our air quality. But not at the cost of jobs.”
Ms. Boyd of the petroleum association said the bill’s sponsors had erred in trying to push the measure through without explaining how it might work. “We think there should be a lot more detail,” she said, “and it should be articulated pretty clearly about how one thinks they are going to be about this super-aggressive mandate.”
Backers of the bill said reductions would be achieved by, among other things, bolstering the fuel efficiency of existing cars and increasing the number of electric cars on the roads, while pushing urban planning policies that help enable people to walk to their jobs and to shopping districts.
“We don’t have a choice — we have to make these changes,” said Tom Steyer, a billionaire hedge fund manager and environmental advocate who has been championing the bill. “In listening to these people talk about how there is going to be rationing, I’m like, ‘Stop making up stories, and start telling us what will happen under your scenario.’ 
“We are in the process of changing how we use energy in the United States of America,” Mr. Steyer added. “The way this happens is, the private sector comes up with new ideas, and people either like them or not.”
Mr. de León, the leader of the State Senate Democrats, said he was preparing amendments to his bill to try to ease concerns. One amendment would give the Legislature more say over the final recommendation by the Air Resources Board.
Even if the bill fails, Mr. Brown said, enough other legislation is already in place that he is confident of long-term victory.
“This is not the whole battle,” he said. “This bill has become a lightning rod. It’s important. But California is way down the road in terms of the thrust and momentum that has been building up for over a decade.”

A version of this article appears in print on September 6, 2015, on page A16 of the New York edition with the headline: Bill to Slash Emission of Carbon Opens Up a California Divide.