Organizers say they are expecting tens of thousands of protestors in Washington Sunday, for what they are calling the biggest climate rally in U.S. history.

It’s primarily to protest the Keystone XL pipeline, which would carry Alberta bitumen to Texas refineries.

On Wednesday, meanwhile, a rally outside the White House led to 48 arrests including Robert Kennedy Jr. and Daryl Hannah.

Those arrests will likely not happen at Sunday’s protest, according to Daniel Kessler of 350.org.

“165 buses coming from 31 different states, so it’s clearly all across the country people are fired up and are coming to tell the president they want action,” Kessler said.

Approving the Keystone XL pipeline is a step in the wrong direction, according to Kessler.

The final decision will come down to U.S. Secretary of State, John Kerry.

Patrick Michaels of the Cato Institute believes the decision has likely already been made, however.

“If they come out against it, they’re going to pay a very, big price,” Michaels said.

Prime Minister Stephen Harper has advocated for the approval of pipeline several times.