Mr. Steyer poured tens of millions of dollars into a successful 2012 ballot initiative in California that eliminated a loophole in the state’s corporate income tax and dedicated some of the resulting revenue to clean-energy projects. He also has helped finance opposition to the Keystone XL pipeline, appearing in a series of self-funded 90-second ads seeking to stop the project.

Those efforts cemented his partnership with Chris Lehane, a California-based Democratic strategist, and heralded the emergence of NextGen Climate, now a 20-person operation encompassing a super PAC, a research organization and a political advocacy nonprofit. The group employs polling, research and social media to find climate-sensitive voters and spends millions of dollars in television advertising to try to persuade them.

It already is among the biggest environmental pressure groups in the country: For example, the League of Conservation Voters, considered the most election-oriented of such groups, reported spending about $15 million on campaign ads in 2012. And while Mr. Steyer has been critical of Democrats who waver on climate issues, he has aimed most of his firepower so far at Republicans.

The new fund-raising push seeks to tap into the booming fortunes of Silicon Valley, where many donors rank climate change as their top political issue. It also signals a shift within the environmental movement, as donors — frustrated that neither Democratic nor Republican officials are willing to prioritize climate change measures — shift their money from philanthropy and education into campaign vehicles designed to win elections.

“There are some people I like and am friends with in the Senate, and if not for Tom’s effort I would probably write a check to support them,” said Wendy Abrams, a Chicago-based philanthropist and donor who raised money for President Obama’s re-election campaign. “But the party is afraid to fight the fight, because they’re afraid to lose more conservative Democratic votes.”

This month, NextGen asked supporters to pick one congressional candidate, from five running this year, for the group to target in its next ads. Four of the five candidates were Republicans, including Senator Marco Rubio of Florida. But the fifth was a vulnerable Democratic incumbent, Senator Mary L. Landrieu of Louisiana, who has close ties to the oil and gas industries and has been an outspoken supporter of the Keystone pipeline.

It is unclear how aggressively his group will move against other Senate Democrats: Asked whether Democratic control of the Senate was necessary to advance his climate agenda, Mr. Steyer said, “As long as we have this partisan divide on energy and climate, it’s got to be important.”