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 350.org, MoveOn.org and Greenpeace USA, signed the missive.