Bryan Templeton is facilities manager at the Keystone facility. The pipes at left are literally the ones that will connect the existing Keystone operation with the new expanded Keystone XL (AKA Keystone B) which is under construction. A massive construction project is well underway on what is known as Keystone B, (AKA Keystone XL) the expansion of the existing Keystone oil pipeline. (Photo by Michael S. Williamson/The Washington Post) The Keystone XL pipeline would transport oil from Canada to the Gulf Coast. (Michael S. Williamson/The Washington Post)

Leaders of more than two dozen influential environmental groups are sending President Obama a letter Tuesday with a clear warning: Even if Canada agrees to reduce the oil and gas sector's climate impact, he should not approve a presidential permit for the Keystone XL pipeline.

The letter, obtained by The Washington Post, provides the strongest evidence yet that president will face a revolt from his political base should he sign off on the pipeline, which would transport heavy crude from Alberta to Gulf Coast refineries. Chief executives of mainstream environmental organizations, including the Natural Resources Defense Council and League of Conservation Voters, along with more left-leaning groups, such as, and Greenpeace USA, signed the missive.

Environmentalists oppose the project on the grounds that it will expand production of an energy-intensive crude that is more damaging to the climate from a region known as the oil sands, or tar sands. Proponents say it will ensure a stable oil supply from one of America's closest allies while creating short-term construction jobs along the pipeline's route.

In an interview Monday, LCV president Gene Karpinski said that he and others welcomed the fact that the Environmental Protection Agency proposed curbing carbon emissions from new power plants last week but that approval of the pipeline would undermine these other climate policy proposals.

"While we take major steps forward, we can’t be taking major steps back at the same time," he said, noting that 75,000 activists have pledged to engage in civil disobedience if Obama approves the permit. "The intensity out there has not diminished one bit. If anything, the willingness of people to go to jail over this is expanding."

In an interview with The New York Times in late July, Obama raised the idea of some sort of carbon offset on Canada's part, saying that Canada “could potentially be doing more to mitigate carbon release” at the pipeline’s source. “We haven’t seen specific ideas or plans,” the president added. “But all of that will go into the mix in terms of [Secretary of State] John Kerry’s decision or recommendation on this issue.” Permitting authority for the Keystone falls under the State Department because the pipeline crosses an international boundary.

"We are pleased to hear reports that Canadian officials may be considering new policies to mitigate global warming pollution from the oil and gas sectors," the 25 environmental leaders wrote. However, on behalf of our millions of members and supporters nationwide, we oppose any deal-making in return for the Keystone XL tar sands pipeline. Our rationale is simple. Building Keystone XL will expand production in the tar sands, and that reality is not compatible with serious efforts to battle climate change."

Michael Brune, executive director of the Sierra Club, is sending a separate letter Tuesday making the same point.

"Mr. President, please do not make a bilateral agreement approving the Keystone XL based on the government of Canada’s mitigation promises," Brune wrote. "While this may seem like a generous offer, Canada simply cannot mitigate the carbon pollution from the pipeline; those emissions would simply be too big. Keystone XL would be directly responsible for the equivalent annual emissions of 51 coal-fired power plants or 37.7 million cars. As a point of comparison, Canada has about 26 million cars on the road."

Earlier this month the Canadian Broadcasting Corp. reported that Canadian Prime Minister Stephen Harper had written a letter to Obama offering to cut the oil and gas sector's greenhouse gas emissions in exchange for Keystone's approval. However it is unclear whether this letter exists: Harper would not confirm it when asked about the matter last week, and Obama administration officials have said they have no knowledge of such a letter.

State Department spokeswoman Jen Psaki said in a statement that the agency is continuing to review the pipeline in a measured manner.

"The State Department’s goal has always been to conduct the review the right way, not in a rushed way," she said. "As part of this review, we are looking at existing information, new information, and responding to public comments. We are committed to a review that is objective, transparent, and rigorous.”