Thomas Steyer, a San Francisco hedge fund manager and a big backer of Democratic candidates, will donate $5 million to a group opposing the ballot measure to roll back California's landmark climate change law.

Steyer, founder of Farallon Capital Management LLC, has joined George Shultz, former U.S. secretary of the state, as co-chairman of the No on 23 committee, giving the group's leadership a bipartisan mix.

California's greenhouse gas reduction law, or AB 32, aims to cut emissions to 1990 levels statewide by 2020.

Backed by Valero Energy Corp. and Tesoro Corp. of Texas, Proposition 23 seeks to suspend AB 32 until the statewide unemployment rate drops to 5.5 percent for four consecutive quarters.

With Steyer's donation, the No on 23 committee has raised more than $7 million. Proponents of a rollback collected more than $3 million.

"Proposition 23 really boils down to one thing," Steyer said in a news release. "Do we want California to continue moving forward as a leader in a clean energy economy, including continuing to create new jobs, new economic development and new investment, or do we want to allow two Texas-based oil companies … to take our state backward and see the clean energy jobs, business and investment in our state go offshore to (a) place like China?"

Steyer founded Farallon Capital in 1986. The firm now manages about $20 billion.

Last year, Steyer and his wife, Kat Taylor, donated $40 million to fund the Precourt Institute for Energy at Stanford University.

Steyer has donated thousands of dollars to Democratic Party candidates, including President Barack Obama, Hillary Clinton, Barbara Boxer, Nancy Pelosi and Dianne Feinstein, according to the Center for Responsive Politics.

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