"By virtue of this letter, NextGen Climate is taking a Courage Pledge to support members of Congress who are now being threatened with political attacks because they had the courage to stand up for our children by opposing the Keystone XL pipeline," Steyer, a former hedge fund manager, wrote in the letter, which was first obtained by Politico.
The letter doesn't say exactly how Steyer plans to support such lawmakers, though he hasn't been shy about his intention to amass a sizable war chest to play in the midterm elections and beyond.
Steyer plans to contribute $50 million of his own money to his PAC and is courting big donors to match that amount. He was a significant spender in the Virginia gubernatorial election last fall, as his PAC spent about $8 million to help elect Democratic Gov. Terry McAuliffe.
The letter comes a week after 11 Senate Democrats sent a letter to President Obama urging him to approve the Canada-to-Texas pipeline. Six of those lawmakers -- Sens. Mary Landrieu of Louisiana, Mark Begich of Alaska, John Walsh of Montana, Mark Pryor of Arkansas, Kay Hagan of North Carolina and Mark Warner of Virginia -- are facing tight re-election contests.
Most lawmakers support the TransCanada Corp.-built pipeline, which is under federal interagency review for a cross-border permit to complete the northern leg, which could be completed this summer.
Keystone XL's union and business supporters say the pipeline would create jobs — the State Department says it would add 42,100 direct and indirect jobs during a two-year construction phase — and strengthen energy security.
Environmentalists fiercely oppose it, as they say it would contribute to greenhouse gas emissions that worsen climate change. A State Department review, however, downplayed Keystone XL's effect on facilitating growth in oil sands, the underpinning of opponents' argument that Keystone would boost emissions.
Steyer has made the pipeline his signature issue, but he has branched out in recent months to become the Left's answer to the Koch brothers.
Democrats have increasingly targeted the conservative billionaires in the run-up to the midterm elections, and Steyer did the same in his letter Thursday.
"We fundamentally believe that those who want to do right by our children and working families must work together for America. We continue to take on powerful forces like TransCanada, the Koch brothers, and climate change deniers in Congress who are seeking to advance their own economic self-interests at the expense of our country," the letter said.