(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