U.S. Vice President Joe Biden is due to attend a fundraiser hosted by Keystone XL opponent Tom Steyer.
The billionaire activist will host the Democratic event at his San Francisco home next week.
Steyer has spent millions of dollars on a lobbying campaign aimed at mothballing the pipeline project.
Supporters say the 1,179 mile stretch of crude oil pipeline is critical to U.S. energy security.
However, opponents say the scheme, which would bring oil from Alberta to the U.S. Gulf Coast, would have devastating effects on the environment.
The pipeline would transport up to 830,000 barrels of oil per day to the Gulf Coast and various Midwest refineries. It would drastically cut the country’s dependency on oil from the Middle East and its South American neighbours.
Earlier this year, the Obama administration extended the review of Keystone XL over a dispute regarding where the route should weave. The move allows the Nebraska Supreme Court and Calgary-based TransCanada more time to resolve the issue.
Biden is a potential Democratic candidate for the 2016 presidential race.