The proposition looks likely to be defeated in part due to the well-funded challengers, as well as the public's displeasure with oil companies after the BP Plc spill this year in the Gulf of Mexico, said Robert Stern, president of the Center for Governmental Studies in Los Angeles.

"There has been a lot of opposition to this," Stern said.

To pass, the proposition requires a majority vote. Among likely voters, 48% oppose Proposition 23 and 32% support it, according to a poll released today by the University of Southern California and the Los Angeles Times. A Sept. 29 poll from the San Francisco-based Public Policy Institute of California indicated a much closer contest, with 42% of likely voters opposed to the ballot initiative and 43% supporting it.

Refiner Fundraising

Tesoro, Valero, and Flint Hills Resources LLC, a refining subsidiary of Wichita, Kansas-based Koch Industries Inc., have raised more than two-thirds of the $10.6 million that has financed support of the proposition. Backers say the measure is needed to prevent job losses and will give California's economy time to recover so that it can better absorb the cost of climate regulations.

Proposition 23 stipulates that the law for cutting greenhouse gases would not take effect until California's unemployment rate falls to at least 5.5 percent for four consecutive quarters. Since 1970, there have been three periods when the state's jobless rate has fallen that low for that long, according to an analysis of the ballot measure by the state's Legislative Analyst's Office, a non-partisan agency that works for the legislature.

'Common-Sense Approach'

Valero, based in San Antonio, has 1,600 employees in the state and remains a "dedicated and enthusiastic supporter" of the measure, Bill Day, a Valero spokesman, said in a telephone interview. "We still think it is a common-sense approach to some of the economic difficulties California faces," Day said.

Tesoro, also based in San Antonio, "firmly supports" the proposition, Lynn Westfall, a Tesoro spokesman, said in an e- mailed statement. Its passage "would be a major milestone in the recovery of the California economy and improve its dismal unemployment rate."

Koch Industries did not respond to requests for comment.

On Oct. 19, 68 investors managing $415 billion in assets, including venture capital firms Kleiner Perkins Caufield & Byers and VantagePoint Venture Partners, issued a statement opposing the measure. Opponents say it may trigger a backlash against government support for alternative-energy sources across the rest of the U.S.