As of June 23, the letter had been signed by 50 members of the House. Many of the signatories sit on the committees for Energy and Commerce, Natural Resources and Transportation and Infrastructure. The letter highlights the fact that the Tar sands mining emit three times more greenhouse gas pollution than traditional oil.

"(This pipeline) cuts through sensitive ecosystems, crosses rivers, invades ranches and farms and could scar this land forever," states Rep. Steve Cohen, one of the lead signatories on the letter. He further stated that the pipeline is slated to pass over the nation's largest underground aquifer, which would leave "irreparable" environmental scars in its wake.

TransCanada, which is building the Keystone and Keystone XL pipelines to transport tar sands bitumen to U.S. refineries, has been pressing for presidential approval of the $12 billion Keystone XL pipeline, which would export up to 900,000 barrels of oil per day.

On July 26, the State Department announced that it was adding 90 days to the comment period for other federal agencies on whether TransCanada Corp.'s Keystone XL pipeline is in the national interest. Two other pipelines have already been okayed by the State Department – the Keystone pipeline, which will eventually carry crude to Cushing, Oklahoma, and the Alberta Clipper pipeline, which runs from Canada to Superior, Wisconsin.

If all three pipelines are built, the tar sands would make up 15 percent of U.S. fuel supply, up from the current four percent today.