A State Department assessment of the pipeline said some of the environmental drawbacks of Canadian oil sands would be problematic with or without Keystone XL. Oil sands are viewed as more corrosive and more carbon intensive than other forms of crude oil though a series of rival studies challenge those claims.
Rep. Henry Waxman, D-Calif., ranking member of the House Committee on Energy and Commerce, and Sen. Sheldon Whitehouse, D-R.I., chairman of a Senate oversight committee on energy policy, sent a 20-page letter to the State Department challenging its assessment of Keystone XL.
The White House said it would consider the project if it does not contribute significantly to carbon emissions. Waxman said he believes the pipeline fails that test.
"The tar sands oil to be transported by the Keystone XL pipeline would wreak havoc on our climate by unleashing up to 22 percent more carbon pollution than average crude oil refined in the U.S.," Whitehouse said in a statement Wednesday.
The letter to Assistant Secretary of State for Oceans, Environment, and Science at the State Department Kerri-Ann Jones contains more than 80 footnotes, including carbon studies from the Environmental Protection Agency.
Keystone XL needs federal approval as a cross-border pipeline. A domestic leg from Oklahoma is under construction.
Supporters of the project say it would provide a source of economic stimulus and ensure North American energy independence.