Tom Steyer
U.S. President Barack Obama looks out as House Democratic Leader Nancy Pelosi (in red) points out toward the ocean during a Democratic fund raiser at the home of billionaire former asset manager Tom Steyer in San Francisco April 3, 2013. Obama will attend four Democratic fund raisers, as part of a fund raising drive for the 2014 U.S. mid-term elections, during his visit to San Francisco. REUTERS/Kevin Lamarque

A San Francisco billionaire who's put hundreds of thousands of dollars into ad campaigns for the environment and against climate change has moved to the forefront of the fight to block the Keystone XL Pipeline.

Tom Steyer, the founder of a $19 billion hedge fund, Farallon Capital Management, is becoming a powerhouse in environmental activism, and recently Politico named him the top 50 “Politicos” to watch.

The proposed Keystone XL pipeline would deliver up to 830,000 barrels per day of crude oil from the tar sands of Alberta, Canada, and the Bakken Shale Formation in North Dakota to refineries in the U.S. Gulf Coast. Environmental groups, like Natural Resources Defense Council, say tar sands are more carbon-intensive to produce than conventional crude oil and that the Keystone pipeline would dangerously hike carbon emissions.

On Tuesday Steyer challenged Russ Girling, the head of the energy company building the pipeline, TransCanada Corp. (TSE:TCA-Y), to a live debate on the project.

"I care so much that the truth comes out that I hereby challenge you to a debate on the merits and faults of the Keystone XL pipeline," Steyer wrote. "Let's have a real, substantial conversation about the issues at hand, and have the viewing public and have the public decide for themselves as to which of us is in possession of the more persuasive and the more important argument," he said.

Steyer left Farallon last year, proclaiming that he did not feel comfortable “being at a firm that was invested in every single sector of the global economy, including tar sands and oil.”

Last week Steyer spent $400,000 on television ads against Virginia’s Republican gubernatorial candidate, Ken Cuccinelli, whom he calls a “climate change denier.”

The San Francisco Democratic activist is also seen as a possible gubernatorial candidate in California next year. He has initiated a social media campaign called “We Love Our Land,” aimed at shedding light on the environmental damage from projects like the Keystone XL pipeline.