Visit the Natural Resources Defense Council's website Monday, and you can expect to find a black screen. More than 400 groups, including the National Wildlife Federation's Action Fund and 350.org, a leader in the fight against the Keystone XL pipeline, are doing the same thing.

Environmental and other groups say they are staging the one-day blackout to protest efforts by Canadian Prime Minister Stephen Harper and his allies to crack down on opponents of the pipeline. Nearly all Canadian environmental organizations, all four opposition parties and others will shroud their websites in black, displaying the message, "While our websites may be dark, our voices together are louder than ever."

Construction of pipelines that would carry petroleum from Alberta's oil sands to the U.S. is a top priority for the Harper administration and his Conservative Party.