A wealthy billionaire who has spent millions on political races around the country plans to try to influence the makeup of the California state Senate by targeting Democratic candidates. It’s not the Koch brothers or any of the members of their influential conservative donor network — it’s Tom Steyer, the environmentalist who has made fighting climate change the cornerstone of his political crusade.

In an interview with the Sacramento Bee’s editorial board, Steyer said he plans to spend $1 million on legislative contests in California, where Democrats hold an almost insurmountable majority. Steyer said some of his targets might be Democratic incumbents.

“There’s nothing in our bylaws that I’ve read that says we can’t get involved in D-on-D races,” Steyer said in the interview. “There are some races in California that are contentious, where there’s a clear choice here.”

Steyer supports expanding California’s cap and trade program to include vehicle fuels, a proposal currently before the state legislature. The oil and gas industry has opposed that expansion, which they say will add significantly to the price of a gallon of gasoline.

In June, 16 state Assembly members signed a letter to the California Air Resources Board opposing the expansion. Just one, Assemblywoman Cheryl Brown (D), faces a fellow Democrat in November.

Five Senate contests will feature two Democratic candidates in November after the June 3 top-two primary. Those include the Sacramento-based 6th District, where Assemblymen Roger Dickinson (D) and Richard Pan (D) finished ahead of two Republicans; and the Hollywood-based 26th District, where activist Sandra Fluke (D) faces Santa Monica-Malibu School Board member Ben Allen (D).

Sens. Ben Hueso (D), Holly Mitchell (D) and Kevin de Leon (D), the incoming Senate president, all face Democratic challengers in November, but all are likely to cruise to reelection.

Steyer has pledged to spend up to $50 million of his own money on the midterm elections this year. He’s made a priority of races in which candidates present a clear difference on climate change, in states like Florida, Colorado and Pennsylvania.