(Bloomberg News) Clean-energy investors and environmentalists in California raised $11.9 million in the past two weeks to snuff out a challenge, backed by oil refiners Tesoro Corp. and Valero Energy Corp., to the state's global-warming laws.

Voters in the most-populous U.S. state will decide in seven days on Proposition 23, a proposal to suspend a state law restricting greenhouse-gas emissions until California's unemployment rate falls to at least 5.5%. The rate in September was 12.4%, third-highest after Nevada and Michigan.

Microsoft Corp. founder Bill Gates, Google co-founder Sergey Brin and James Cameron, director of the world's top- grossing film "Avatar," have donated to the campaign in the past two weeks, according to state records. If passed, the measure would undermine the nation's largest solar market and threaten $9 billion in venture capital investments, according to analysts, investors and renewable-energy companies.

It would have "a significantly negative impact on the valuation of solar energy stocks," said Ramesh Misra, a solar analyst with New York-based Brigantine Advisors.

First Solar Inc., the world's biggest maker of solar panel modules, SunPower Corp., the second-biggest U.S. supplier of solar modules, and Yingli Green Energy Holding Co., China's second-largest maker of solar panels, may fall if the measure passes, Misra said.

Share Prices

Shares of First Solar, based in Tempe, Ariz., rose 7.5% this year through Oct. 22. SunPower, based in San Jose, Calif., fell 43%. Yingli Green Energy's American depositary receipts dropped 26%. Each ADR represents one ordinary share.

Groups opposed to the ballot initiative have taken in more than $30 million to sway voters with radio, television and print advertising, out-raising supporters of the measure by almost three to one, according to state records.

The proposition would delay enforcement of California's Global Warming Solutions Act, which was signed into law by Governor Arnold Schwarzenegger in 2006 and requires the state to cut output of greenhouse gases linked to climate change to their 1990 levels by 2020. The carbon law would create a market for carbon dioxide pollution permits and require utilities to buy almost a third of their electricity from renewable sources such as solar panels.

Fading Support

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.

"I know it will have a national effect," Jim Watson, a San Francisco-based venture capitalist who serves on the executive committee of the "No on 23" campaign, said in an interview in Washington. "It really is a test of the people's will," said Watson, the managing general partner of CMEA Capital, which has $1.2 billion invested in energy, information technology and life sciences companies, according to its website.

Private Contribution

Gates's contribution was a private one and not from the Bill and Melinda Gates Foundation, said John Pinette, a spokesman for Gates. Google declined to comment on Brin's contribution. Cameron declined to comment on his donation, said Steven Maviglio, a spokesman for a committee that is campaigning to defeat the proposition.

Backers of the ballot initiative are "quite confident" it will prevail on election day, Anita Mangels, a spokeswoman for the "Yes on 23" committee, said in a telephone interview.

"The volume of venture-capital dollars" that have been devoted to defeating Proposition 23 are meant to "artificially prop up" investments in "clean-tech" companies, Mangels said.

Venture capital firms have invested $9 billion in clean- technology companies in the state since 2005, said Martin Lagod, co-founder and managing director of Firelake Capital Management LLC in Palo Alto, California and an opponent of the ballot measure.

Most of that money was invested on the assumption that California would enforce its greenhouse gas limits, he said.