Opponents of a controversial pipeline from the Alberta oilsands to the U.S. Gulf Coast are pushing Secretary of State Hillary Clinton to order another environmental impact study of the project.

A letter signed by 28 members of Congress, who say the original State Department study was deeply flawed, was released Tuesday by the No Tar Sands Coalition, which includes the Natural Resources Defence Council.

The coalition said earlier Tuesday that it had launched a $500,000 ad campaign to try to persuade U.S. President Barack Obama to reject the controversial Keystone XL pipeline.

The pipeline would be built by TransCanada Corp. of Calgary, which has one of North America's largest networks of energy pipelines.

The company has said the pipeline is necessary to provide the United States with a dependable source of oil and insisted it will be environmentally responsible.

The politicians' letter, dated Dec. 3 and signed by members of the House of Representatives, opposes the Keystone XL pipeline and the oilsands as a whole on environmental grounds.

Among the signatories were Steve Cohen, who sits on the House committee that oversees U.S. pipelines, and Jay Inslee, who is on the House committees that deal with energy and natural resources.

The letter says the politicians are concerned about Clinton's statement the U.S. State department is "inclined to approve" the Keystone project.

"The Environmental Protection Agency (EPA) found the State Department's assessment to be inadequate and asked that a new EIS be conducted," the letter said. "In fact, EPA gave the draft EIS its lowest possible rating."

TransCanada Corp. needs a permit from the State Department before it can begin construction on Keystone XL, which would stretch 3,200-kilometres from Hardisty, Alta., to Port Arthur, Texas.

Keystone XL is an expansion to a TransCanada pipeline that runs from Alberta to refineries in the U.S. Midwest. The pipeline would go through Montana, South Dakota and Nebraska before running down to the U.S. Gulf Coast, where the bulk of U.S. refining capacity is located.

The Dec. 3 letter follows a similar letter in June, prior to the midterm elections in November, signed by 50 members of the House of Representatives, including Cohen.