The U.S. Environmental Protection Agency is challenging the basic logic cited by project supporters, which includes the Canadian government and the oil industry.
A pillar of the argument for this project is that Canada's oil industry will continue growing anyway — so it might as well use pipelines, which are a cleaner transport method than trains.
But the EPA says falling oil prices mean it's important to reconsider that assumption about growth of Canada's oilsands.
In a letter to the State Department, which is leading the pipeline review, the environmental agency says its colleagues should give more weight to a long-term, low-oil-price scenario. It compares the carbon emissions from the pipeline to 5.7 million new passenger vehicles on the road.
The State Department is finishing its long-awaited, oft-delayed review and could make a recommendation within weeks to President Barack Obama.