SACRAMENTO, Aug. 30 — California’s political leaders announced an agreement on Wednesday that imposes the most sweeping controls on carbon dioxide emissions in the nation, putting the state at the forefront of a broad campaign to curb the man-made causes of climate change despite resistance in Washington.

The deal between the Democratic-controlled Legislature and the Republican governor, Arnold Schwarzenegger, calls for a 25 percent reduction in carbon dioxide emissions by 2020, and could establish controls on the largest industrial sectors, including utilities, oil refineries and cement plants. The state has already placed strict limits on automobile emissions, although that move is being challenged in federal court.

The Bush administration has rejected the idea of similar national controls on carbon dioxide emissions, and efforts to get Congressional approval for such firm caps on emissions have repeatedly been defeated.

Although the deal in California is strongly opposed by Republicans in the Legislature and many business leaders across the state, it assures that a bill on the restrictions will be passed before the legislative session ends Thursday and will be signed by Mr. Schwarzenegger, the leaders said Wednesday.

The first major controls are scheduled to begin in 2012, with the aim of reducing the emissions to their level in 1990. The legislation allows for incentives to businesses to help reach the goals, but opponents warn that the state may be sacrificing its economic interests for a quixotic goal.

“If our manufacturers leave, whether for North Carolina or China, and they take their greenhouse gases with them, we might not have solved the problem but exacerbated it instead,” said Allan Zaremberg, the president of the state’s Chamber of Commerce.

Since taking office in 2003, Mr. Schwarzenegger, who is seeking reelection in November, has supported efforts to fight climate change, most recently by signing an agreement with Prime Minister Tony Blair of Britain to do cooperative research on new clean-energy technologies.

The governor said Wednesday that the deal struck with Democrats would make “California a world leader in the effort to reduce carbon emissions.”

“The success of our system will be an example for other states and nations to follow as the fight against climate change continues,” Mr. Schwarzenegger added.

The Assembly speaker, Fabian Núñez, who sponsored the bill along with Assemblywoman Fran Pavley, said at a news conference here, “We feel that California has always been a leader in protecting the environment.”

“We now have moved it to the next level,’’ said Mr. Núñez, a Los Angeles Democrat. “We’d all like to see California one day be carbon free.”

The state’s action, he said, could set off a “bottom-up” movement for curbs of heat-trapping gases in states around the country.

That has been the goal of national environmental groups like Environmental Defense and the Natural Resources Defense Council, which helped sponsor the California legislation. It has also been the goal of Ms. Pavley, a Democrat from Agoura Hills and the author, in 2002, of a groundbreaking law reducing tailpipe emissions of heat-trapping gases.

Already, the governors of New York, New Jersey, Delaware and four New England states have signed an agreement to curb power-plant emissions, cutting them by 10 percent by 2019. That would amount to about 24 million tons, said Dale Bryk of the New York office of the Natural Resources Defense Council, about one-seventh of the total envisioned in the California legislation, which will be an estimated 174 million tons.

Ms. Bryk added that Massachusetts and New Hampshire had enacted some emissions curbs, but that the number of power plants involved was minuscule compared with the California effort.

A recent poll of Californians by the Public Policy Institute of California showed nearly four of five respondents said urgent action on climate change was needed.

Aside from its long coastline, which could be vulnerable to sea-level rises due to global warming, the state depends on the Sierra Nevada snow pack for much of its water.

A study in 2004 by the National Academy of Science showed that unchecked global warming would cut the size of the snow pack by at least 29 percent by the end of the century. It also predicted a doubling in the number of heat waves, like the record-breaking one in July that killed 139 people statewide.

The deal on Wednesday on the emissions legislation nearly foundered at least three times in the past week as Mr. Schwarzenegger’s negotiators reached seeming impasses with the Legislature over important issues: whether the bill would require the creation of market mechanisms like emissions credits to help industries meet the new standards; how broad to make exemptions during emergencies like the state’s electricity crisis six years ago, and how to administer and enforce the law.