The domestic oil boom in the U.S. is getting a big boost from a major new supply of crude imported from the friendly neighboring nation north of the border. But Canadian crude from the oil sands of Alberta—or tar sands, as environmentalists call it—also comes with a heavy environmental cost, both in terms of local water pollution and greater carbon emissions. That led greens to oppose the construction of the Keystone XL pipeline, which would have transported hundreds of thousands of barrels of oil sands crude down to the U.S. At the same time, Republicans—and more conservative Democrats—were pushing for the pipeline as another step towards continental energy independence. In the end, President Obama decided to block the project temporarily—but with re-election assured, he may take a second look at Keystone.