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Tens of thousands of protesters marched on Washington Sunday, in what organizers say was the largest climate rally in US history. They had varying agendas but unified on one point: Blocking construction of the Keystone XL pipeline.

“When you are in a hole, stop digging,” Bill McKibben, cofounder of the environmentalist group 350.org,, told the crowd gathered on the National Mall. “Above all, stop the Keystone [XL] pipeline. The president can do that ...