Over 35,000 people descended on the National Mall in Washington on Sunday, huddled together against a stinging cold wind to deliver a message of opposition to the Keystone XL pipeline. Their audience was really just one man, the only one with the power to stop the project: Barack Obama.

“This movement has been building for a long time. And one of the things that’s built it is everybody’s desire to give the president the support he needs to block this Keystone pipeline,” Bill McKibben, president of 350.org, told reporters just before the rally began.

“The time for him to stand up is now. He’s been saying good things about climate change, but the easiest, simplest, purest action he could take is to not build this long fuse to one of the biggest carbon bombs on earth.”

That message of constructive support is evident in the rally’s title, the “Forward on Climate” march, which co-opted the president’s campaign 2012 campaign slogan.

The famous Obama “O” from campaign signage is also part of the rally’s graphic messaging, and numerous placards with inspirational quotes from Obama about climate change could be seen in the crowd. Several speakers, including U.S. Senator Sheldon Whitehouse, addressed the crowd before a March to the White House.

So this isn’t your standard protest, but rather a rally of support. But organizers aren’t naïve about political realities—and there was some tough talk for the president, too.

Van Jones, who worked in Obama’s White House in 2009 as his “green jobs czar,” made it clear in a pre-rally interview that the burgeoning anti-pipeline movement would not be bought off with other initiatives, like tougher EPA rules or more great speeches.

“I think we should take the president at his word, but make him honor his word,” Jones said. “This pipeline, if it goes through—the first thing that the pipeline runs over is the credibility of the president of the United States. That’s the first thing it runs over. He said that he’s not going to let us be a generation that cooks the earth.”

Jones continued: “If we lose, we lose everything. We’re fighting for the children of all species. This isn’t just a fight about Democrats versus Republicans in the United States. The children of all species forever are going to be impacted by what we do in this town for the next twelve to twenty-four months.

“So when you have that kind of fight, and people know that’s the fight that it is, you’re not going to see the sort of compromising you saw in the past. This is a different environmental movement. This is a different moment.”

And if Obama does approve the pipeline? Jones said it would define Obama in history’s eyes.

“Every other gain this president has done will be erased over the next ten, twenty, thirty years by floods, by fires, by droughts, by superstorms. His legacy is on the line.”