But Keystone XL's international route makes challenging it more complicated. The State Department says environmental impacts in Canada aren't within its jurisdiction, and the courts may agree. "(They) usually don't apply the National Environmental Policy Act (NEPA) to activities in other countries," says Dan Farber, an environmental law professor at the University of California-Berkeley. "On the other hand, climate change might be different because the ultimate harm will involve the U.S. environment."

At least one federal court has required that cross-border emissions be considered. In 2003, a U.S. District Court in California said transmission lines running into the state from Mexican natural gas plants were permitted illegally because the environmental analysis didn't consider the impacts of carbon and ammonia emissions from generating the electricity in Mexico.

Still, so far environmentalists haven't been successful in stopping or forcing new environmental reviews for two tar sands pipelines that were permitted, one under President Bush and the other under Obama.

Will this time be any different? Maybe. The permitting decision hinges on whether the State Department deems the project in the "national interest," but there are no set criteria for making that call. "Every pipeline is a whole new game," says Susan Casey-Lefkowitz, director of international programs for NRDC.

While the legal context hasn't changed much since the last pipeline was permitted, the political landscape may have. House Committee on Energy and Commerce chair Henry Waxman, D-Calif., recently wrote a 10-page letter chiding the State Department for ignoring the project's "most significant environmental impacts" -- those in Canada. And EPA's comments on the draft EIS last month echoed environmentalists' cross-border concerns. Full disclosure of the pipeline's impacts must include "extraction-related greenhouse gas emissions," the agency scolded -- an approach consistent with climate guidelines drafted this year by the Council on Environmental Quality, which oversees NEPA's implementation.

In response, the State Department has delayed its decision while it consults other agencies. That's far from a sure sign that it will deny TransCanada its Gulf Coast ambitions -- the draft EIS more or less dismisses that option, and the department has favored energy security over environmental protection in the past. But State's hedge is evidence that the opposition to Keystone XL isn't just token.

The Albertan government is taking it seriously. It touted the virtues of the tar sands and what would be the industry's biggest pipeline in a $55,800 Washington Post ad this summer, a not-so-subtle reminder that the American oil habit is far from kicked. "A good neighbour lends you a cup of sugar," the ad began. "A great neighbour supplies you with 1.4 million barrels of oil per day."

See a sidebar to this story, "Monstertruck alley."