SACRAMENTO — With just two days to go before the end of the legislative session, Gov. Jerry Brown and Democratic legislative leaders suffered a pair of stunning defeats at the hands of Republicans, oil companies and even some members of their own party.

Shortly after negotiations over a proposed tax hike on gasoline collapsed because of a lack of support from the GOP, Democrats surrendered to oil companies’ demands to amend a bill that seeks to combat climate change, easily marking the worst day of Brown’s fourth term.

Speaking at a Capitol news conference late Wednesday, Senate President Pro Tem Kevin de León announced amendments to Senate Bill 350 that delete a key goal to cut Californians’ use of petroleum in gasoline 50 percent by 2030, conceding that he had been unable to compete with the oil industry’s “bottomless war chest.”

“I don’t think we’ve seen an amount of money spent like we have seen in the last four months — tens of millions of dollars spent to create this smoke screen” of deception with high-priced, negative advertising, he said.

Kathryn Phillips, executive director of Sierra Club California, went so far as to say that the Western States Petroleum Association, the oil industry’s main trade group, had “declared war on humanity” by sabotaging lawmakers’ efforts to pass the bill in its original form.

“The oil companies are ruthless,” she said. “They are determined to tell every lie they can and to scare people to death just so they can keep as much market share as possible.”

Catherine Reheis-Boyd, the group’s president, said she remains committed to working with Brown and legislative leaders on climate change and energy policy.

“Today’s announcement was an acknowledgement that California’s energy future, economic competitiveness and environment are inextricably linked,” Reheis-Boyd said in a statement. “Californians are best served by inclusive energy policy and by a legislative body that retains authority on issues so critically important to jobs, communities and our way of life.”

The association also sought amendments to the bill that would strip power from the California Air Resources Board, but Brown wouldn’t give in and opted to delete the petroleum reduction target from the measure instead. He noted that the state is on track to meet that goal anyway because of high fuel-economy standards enacted by the Obama administration that are set to take effect in the coming years.

The governor told reporters that Big Oil had only won a skirmish, not the war, and that he was more committed than ever to tackling climate change. He also stressed that two other components of the bill — goals to boost the state’s use of renewable energy and strengthen building standards — remain intact.

The warming planet “is an existential threat to our children,” Brown said. “Over the next three years, you’re going to see a continuous effort to ratchet-up the state’s commitment to clean-up our air and reduce carbon pollution, thereby setting an example for the rest of the world on how to save us before it’s too late.”

But some political analysts faulted Brown for not doing enough early on to allay the concerns of some Assembly Democrats that the measure would harm the low-income communities they represent. In the end, it was their opposition that forced Brown, de León and Assembly Speaker Toni Atkins to decide to amend de León’s bill.

“If you’re going to get the Legislature to vote for a policy change this big, you need to put in weeks if not months of steady and patient effort,” said Dan Schnur, a former Republican strategist who directs the University of Southern California’s Unruh Institute of Politics. “You can’t try to jam them up at the end.”

Assemblyman Henry Perea, a leader of moderate Democrats from the Central Valley who supports the amendments, called them a compromise that rightly takes regional concerns about the measure into account.

“I represent a district with very little mass transit,” Perea said. “The idea of halving petroleum really worried my constituents. They told me so every Thursday, Friday, Saturday and Sunday when I returned to my district, and I view these amendments as a victory for them.”

Regarding the tax proposals supported by Brown and many other Democrats, Senate Republican leader Jean Fuller said Wednesday that Republicans can’t support higher taxes on gasoline, tobacco or cocktails — a new tax introduced Tuesday that would provide funding for Californians with developmental disabilities — because Democrats have done a poor job managing the state’s surging revenue.

“At some point, you have to say ‘enough is enough.’ This is a matter of setting priorities,” Fuller said. “Republicans believe taxpayers have provided more than enough money to fund the programs we believe are essential to keeping our state running and protecting our most vulnerable residents.”

Brown still hopes to win Republican votes for a new tax on managed health care plans, but convincing Republicans to back a new health care tax won’t be any easier than getting them to support a transportation tax or fee, political experts say.

“The guy who was governor during Proposition 13 ought to know better than anyone the power of tax issues,” said Jack Pitney, a political expert at Claremont McKenna College, alluding to California’s landmark 1978 ballot measure. “If anything, Republicans have become even firmer over the years on that issue.”

Brown first called on Democrats and Republicans to work together to tackle the state’s $59 billion backlog of roadway maintenance eight months ago. But it wasn’t until late last week that members of his administration met with Republican leaders and pitched the framework of a plan to pay for the repairs.

The governor’s infrastructure plan seeks to raise $3.5 billion annually by boosting the gasoline tax six cents per gallon, increasing the diesel tax by 11 cents and increasing the vehicle registration fee by $65. His plan is a watered-down version of a bill authored by Sen. Jim Beall, D-San Jose, that has already cleared the Senate’s transportation committee. Still, Republicans wouldn’t budge.

Because tax and fee increases require the support of two-thirds of lawmakers in the Senate and Assembly, Democrats seeking to raise taxes need some GOP votes since Democrats lost their supermajorities last year in both houses of the Legislature.

A $2 per-pack tax on cigarettes has also stalled, but a group of doctors and health care providers have vowed to put the issue on the ballot in 2016 if the Legislature fails to act.