Hours before CNN's Democratic climate forum Wednesday evening, several green groups shared that just nine of the current Democratic presidential candidates have signed the "No KXL Pledge," promising to reverse President Donald Trump's permit for the construction of the Keystone XL tar sands pipeline.

Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.), who who have seen their polling numbers rise recently, were two of the first candidates to sign the pledge after it was distributed by groups including Bold Nebraska, 350 Action, and the Indigenous Environmental Network on August 13, but former Vice President Joe Biden, who has been labeled the frontrunner since before the primary race began, has yet to affirm his support.

Critics of the pipeline called on CNN to ask Biden about his failure so far to sign the pledge during Wednesday evening's forum.

Warren and Sanders are joined by Sens. Cory Booker (D-N.J.) and Kamala Harris (D-Calif.), Mayors Pete Buttigieg and Bill DeBlasio, billionaire Tom Steyer, former Rep. Joe Sestak, and former HUD Secretary Julian Castro in signing the agreement, pledging to revoke Trump's permits for the Keystone XL and Dakota Access pipelines on Day One of their potential administration.

"U.S. presidents are not empowered to write up proclamations that give Big Oil a free ride, and bypass our nation's bedrock environmental laws written to protect our water, land, clean air, and a livable climate," read the letter the groups sent to candidates in August. "You may recall that this same pipeline project was rejected in 2014 by President Obama, whose administration found it to be not in the U.S. national interest because it would significantly exacerbate our climate crisis."

Washington Gov. Jay Inslee, who left the race last month after making the climate crisis the central focus of his campaign and influencing Warren and other contenders to adopt some of his proposals for their own plans, also signed the pledge.

The signers also agreed to direct all federal agencies to reject any project that would contribute to fossil fuel emissions or other factors that make the climate crisis worse, including pipeline construction.

The Keystone XL pipeline would carry 830,000 barrels per day of tar sands oil 1,179 miles from Alberta, Canada to refineries in the Gulf Coast. Tar sands oil spills are especially difficult to clean up, and even without spills the oil would causes major health risks to communities it runs through due to the buildup of chemicals and air pollution. The oil would cross over bodies of water and farm land, threatening the Ogallala Aquifer, which irrigates a third of U.S. farms and provides drinking water to millions of people.

In addition to Biden, high-profile candidates including Andrew Yang, Tulsi Gabbard, and Beto O'Rourke have not yet pledged to revoke Trump's permit.

"Keystone XL Pipeline is a political risk," said Jane Kleeb, founder of the progressive group Bold Nebraska. "Especially since most of the [major] Democratic candidates for president are on the record that they would reject KXL Pipeline and put a climate test on ALL fossil fuels."