What is the target galvanizing these forces? The proposed Keystone XL Pipeline, to be built by TransCanada, which would carry crude oil extracted from the tar sands in Alberta, Canada, some 2,000 miles south to the Gulf of Mexico for export.

What is the outcome hoped for by those who will attend? That President Obama reject the project, finally and definitively, when it comes up for his approval this winter.

Protests, blockades, arrests and disruptions of the pipeline's construction have been ongoing by activists and landowners in East Texas since last summer. Now, the "Forward On Climate" rally marks a huge step forward to enlarge the movement. This is not your typical environmental protest in defense of a limited ecosystem.