Pipeline company TransCanada submitted an application to build the cross-border pipeline more than five years ago. President Obama said he'd weigh the project against its environmental footprint, and a State Department report this year said the pipeline would have few net issues.
"The myth that tar sands development is inevitable and will find its way to market by rail if not pipeline is a red herring," they said in their letter published Tuesday. "Oil industry projections are clear that to reach their production goals they would need all current pipeline proposals as well as rail; not either or."
There was no statement from the State Department or the White House regarding the letter.
Supporters of Keystone XL say it would add a layer of security to the North American energy sector while providing a source of economic stimulus. Opponents say the type of oil, a more viscous form dubbed oil sands, carries too many environmental risks.