Blocking the Keystone XL pipeline may be the green movement's top talking point, but it isn't environmental groups' top political priority.

The Sierra Club and the Natural Resources Defense Council's political fund are backing Sen. Kay Hagan of North Carolina, who is locked in a tough reelection fight—and who favors the pipeline. The NRDC Action Fund is also throwing its weight behind Sen. Mark Begich, a pro-Keystone Democrat trying to hold on to a fiercely contested seat in Alaska. And last year, according to the Sunlight Foundation, the League of Conservation Voters contributed thousands of dollars to both.

Why? Both senators back the Environmental Protection Agency's regulations that require power plants to cut their carbon emissions—President Obama's most powerful tool left in the fight against climate change.

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