In a letter to U.S. Secretary of State Hillary Clinton made public Tuesday, Rep. Henry Waxman, D-Calif., chairman of the House Committee on Energy and Commerce, said the pipeline would increase the amount of tar sands oil from Canada to the United States to 3 million barrels a day. That would also increase greenhouse gas emissions, equivalent to putting 18 million more cars on the road, he said.
The process of extracting oil from oil sands -- or tar sands -- is known for releasing higher levels of greenhouse gases than conventional oil drilling.
"This pipeline is a multibillion-dollar investment to expand our reliance on the dirtiest source of transportation fuel currently available," Waxman said in the letter. The State Department's decision whether to permit the project "represents a critical choice about America's energy future," he wrote.
Because the pipeline would cross international borders, the State Department is required to conduct a National Interest Determination and is likely to make a decision in the fall.
Waxman's letter follows another letter from 50 congressmen sent last month to Clinton, citing concerns the pipeline would double the country's consumption of crude from the oil sands.
The lawmakers urged the Obama administration to do a full assessment of the environmental impacts of the oil sands before allowing the project to go ahead.
"It is indicative of a growing wave of opposition," said Kate Colarulli, director of the Sierra Club's dirty fuels program, The Globe and Mail newspaper reports. "Having people such as Waxman expressing their concern, we could see a domino effect."
But some supporters of the pipeline argue that the United States must choose between buying crude from Canada or from nations considered less friendly.
"Where would they have a preference for that oil to come from? Iran? Algeria? Angola?" asked former Alberta Energy Minister Murray Smith, who has lobbied for the Canadian province in Washington.
"Twenty percent of their imports today are from countries openly hostile to United States' interests. Good pipes make for good neighbors," said Smith.
The proposed pipeline would go right past the Gotschall family ranch -- now in its fourth generation of operation -- in Nebraska's Sand Hills.
"Where I'm sitting, having an oil pipeline in my back yard, in my drinking water, that's not a very friendly neighbor," Ben Gotschall told NPR.