Tom Steyer, the California billionaire who has spent more than $35 million of his own funds to back progressive ballot initiatives in the state, signed a letter along with a handful of Massachusetts climate change activists calling for Lynch to either oppose the pipeline's construction, or get a sworn statement from the pipeline's backers that all of the oil shipped via the pipeline will stay in the U.S.

"If you can’t or won’t do either, then you’ll be showing us that you stand with Republicans and a wealthy foreign oil company and against solving the climate crisis. You’ll be standing for the wrong energy future for America," the letter reads.

If Lynch refuses to fulfill the group's request, "we will then immediately launch an aggressive public education campaign," which will include reports released on Lynch's record, get-out-the-vote efforts and public events targeting Lynch, among other activities.

Lynch has voted to pressure President Obama to expedite a decision on the pipeline, which environmentalists says will harm the environment with little advantage to the U.S., as much of the oil is likely to be exported.

His Democratic primary opponent, Rep. Edward MarkeyEdward (Ed) John MarkeyElection Countdown: O'Rourke goes on the attack | Takeaways from fiery second Texas Senate debate | Heitkamp apologizes for ad misidentifying abuse victims | Trump Jr. to rally for Manchin challenger | Rick Scott leaves trail to deal with hurricane damage Senate Dems ask Trump to disclose financial ties to Saudi Arabia Dems damp down hopes for climate change agenda MORE (D-Mass.) is known as a leader on climate change legislation on the House. Markey has already received the backing of the League of Conservation Voters and 350 Action, two prominent climate change groups.

Steyer's spokesman, Chris Lehane, said Steyer had not set a limit on how much he would spend on the race, but that the effort would be largely paid for with his own funds.

"He plays to win," Lehane said. "Obviously since the evil empire, i.e. Big Oil, is on the other side, he's willing to invest [funds]."

Lynch spokesman Conor Yunits characterized the letter as "something out of a James Bond film — a billionaire making threats and issuing ultimatums that expire at 'high noon.'"

"Like President Obama, Congressman Lynch supports an all-of-the-above energy strategy, and he is not going to respond to threats and ultimatums," he said.

But when pressed on whether Lynch would change his mind on the pipeline, Yunits said that he would only do so "if President Obama and Secretary Kerry ultimately decide that this pipeline should not be built."

"But he supports the pipeline," Yunits added.