President Barack Obama should reject the Keystone XL oil pipeline before heading to Paris next month to finalize a global climate agreement, Democratic presidential candidate Bernie Sanders said Wednesday. 

The Vermont independent senator said rejecting the pipeline now would show "bold leadership" in the fight to slow climate change, which Sanders called "a major, major, major planetary crisis." 

The pipeline's developer, Calgary-based TransCanada, has asked U.S. officials to delay consideration of the Alberta-to-Texas pipeline, potentially putting off a decision until the next U.S. president takes office in 2017. 

Sanders, who is challenging former Secretary of State Hillary Rodham Clinton for the Democratic presidential nomination, said Obama should ignore TransCanada's request and act decisively to reject the long-delayed pipeline, a flashpoint in the global debate over climate change. 

The 1,179-mile pipeline would run from Canada through Montana, South Dakota and Nebraska, where it would connect with existing pipelines to carry more than 800,000 barrels of crude oil a day from tar sands in western Canada to refineries along the U.S. Gulf Coast. 

Opponents say the project requires huge amounts of energy and water and increases greenhouse gas emissions blamed for global warming. They also warn that pipeline leaks could potentially pollute underground aquifers that are a critical source of water for farmers in the Great Plains. 

Supporters say the project will create jobs and reduce U.S. reliance on Middle Eastern oil. They argue that pipelines are a safer method of transporting oil than trains, pointing to recent derailments on both sides of the border, including a 2013 disaster in Canada that killed 47 people. 

Sanders, appearing at a rally at the U.S. Capitol sponsored by climate activists, said rejecting the pipeline would signal to the rest of the world that the United States was "walking the walk" on climate change. 

"Clearly, (climate change) is an international crisis. We can't do it alone" Sanders said, singling out China, Russia and India as key players in the fight to slow global warming. 

"But it's important for those countries to understand that we are not just talking the talk, but we are walking the walk" on climate change, Sander said. "And we are going to lead." 

Sanders said he had "zero doubt" that if a Republican wins the presidential election, "on day one the Keystone people will be back pushing for that pipeline. I think their hope is that Republicans win, and when they do the path will be open for that pipeline and other disastrous environmental legislation." 

The GOP presidential field unanimously supports Keystone, while all the major Democratic candidates oppose it, including Clinton, the Democratic front-runner who oversaw the early phase of the pipeline review as Obama's first-term secretary of state.