Bill McKibbin, 350.org. cc Photo by Joshua Lopez - Project Survival Media.

In a call with media Friday morning, organizers of this weekend's "Forward on Climate Rally" in Washington, DC, offered a preview of the event and stressed the critical importance of action by the Obama administration to block the Keystone XL pipeline, the rally's central focus.

"This will be the largest climate rally ever in this country," said Bill McKibben, founder of 350.org. He predicted attendance of around 20,000 people, with 150 buses from 31 states converging on DC for the event, scheduled to kick off Sunday at noon near the Washington Monument.

The four-hour event was organized by a coalition including the Sierra Club, 350.org, Hip Hop Caucus, Environment America, League of Conservation Voters, and scores of other progressive organizations, and will feature speakers -- including RI Sen. Sheldon Whitehouse -- and a "human pipeline" threading down 15th Street to the White House.

McKibben hoped strong turnout would prompt the administration to "do something substantive," about the threat posed by the high-carbon oil the pipeline would bring. "We just learned this week that Arctic ice volume dropped 80% since 1980. This is no time for half measures. If we're serious about climate change, we need to start leaving carbon in the ground."

Using Keystone oil would be "lighting a fuse on a carbon bomb," said Van Jones, leader of "Rebuild the Dream" and a former Obama advisor. "I know the passion this President has for this issue, and how tough the politics are," Jones said, noting that accountability for the decision would ultimately rest with Obama.

"Canceling the Keystone XL pipeline would be a powerful legacy," McKibben added.

Reporter Matt Wald of the New York Times posed a question about the effectiveness of unilateral US action. "Canada is a foreign country," he said. "What makes you think they won't just ship it to their west coast?"

McKibben cited developments over the last year, as Canadian activists have worked to block other options, producing constraints that would impact financial viability. "It's clear now that Keystone XL is the last option," he said.

Susan Casey-Lefkowitz, of the Natural Resources Defense Council agreed, adding that this was the first major tar sands pipeline to deep water. "Developing tar sands depends on Keystone XL," she said.