WASHINGTON (Reuters) - More than 450 groups on Monday called on Congress to reject the Trans-Pacific Partnership if it comes up for a vote this fall, saying the trade deal would allow fossil fuel companies to contest U.S. environmental rules in extrajudicial tribunals.

The groups, most of them environmental organizations, warned that companies could challenge U.S. environmental standards in tribunals outside the domestic legal system under provisions of the 12-nation TPP and the proposed Transatlantic Trade and Investment Partnership (TTIP) with Europe.

Congress is expected to vote on the TPP after the Nov. 8 election during a lame-duck session. President Barack Obama wants the agreement ratified before he leaves office on Jan. 20, but opposition to the deal has grown during this year’s presidential campaign.

“We strongly urge you to eliminate this threat to U.S. climate progress by committing to vote no on the TPP and asking the U.S. Trade Representative to remove from TTIP any provision that empowers corporations to challenge government policies in extrajudicial tribunals,” the groups wrote in the letter to every member of Congress.

Obama’s political ally and Democratic presidential candidate Hillary Clinton has said she wants to renegotiate the TPP to include stronger rules on currency manipulation.

Voter anxiety over the impact of trade deals on jobs and the environment has helped power the campaigns of Donald Trump, the likely Republican nominee, and U.S. Senator Bernie Sanders, who is running against Clinton for the Democratic nomination.

The letter says approving the deals would enable fossil fuel companies to use “investor-state dispute settlements” to demand compensation for environmental rules through cases decided by lawyers outside the U.S. judicial system.

The groups noted that in January, Canadian energy company TransCanada asked for a private tribunal through the North American Free Trade Agreement to seek compensation exceeding $15 billion, after Obama last year rejected a permit for its Keystone pipeline, citing global warming concerns.

“The TPP and TTIP would more than double the number of fossil fuel corporations that could follow TransCanada’s example and challenge U.S. policies in private tribunals,” the letter said.

A spokesman for the United States Trade Representative said on Monday that the United States has never lost an ISDS case because “we have continued to raise standards through each agreement.”

The spokesman said the TPP has the strongest environmental safeguards of any trade agreement and upgrades NAFTA with “full enforceable environmental obligations.”

Ilana Solomon, director of the Sierra Club’s trade program, said skepticism around trade deals in the U.S. election creates an opportunity for Congress to reject the TPP.

“There is so much momentum now to end the TPP and other trade agreements,” she told Reuters. “This is an area where there is bipartisan agreement... that these deals harm workers, communities and our environment.”