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 MarkeyHillicon Valley: Mueller indicts Russians for DNC hack | US officially lifts ZTE ban | AT&T CEO downplays merger challenge | Microsoft asks for rules on facial recognition technology | Dems want probe into smart TVs Dems push FTC to investigate smart TVs over privacy concerns Hillicon Valley: Hacker tried to sell military docs on dark web | Facebook fined over Cambridge Analytica | US closer to lifting ZTE ban | Trump, Obama lose followers in Twitter purge | DOJ weighs appeal on AT&T merger 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.