Canadian pipeline company TransCanada wants to build Keystone XL to supplement its existing pipeline carrying tar sands oil from Alberta. The current network terminates in Oklahoma and Illinois. The extension would end along the Gulf of Mexico.
Critics say they worry about the potential environmental impact of heavy oil from Canada. There have been several spills on the existing Keystone pipeline.
James Little, president of the Transport Workers Union, and Larry Hanley, president of the Amalgamated Transit Union, in a joint statement called on the U.S. State Department to back away from the project, saying it would hurt the U.S. job market.
"We are also concerned that Keystone XL could double the amount of highly toxic tar sands oil being imported into the United States," they said.
Tar sands are viewed by critics as more dangerous than conventional crude oil.
The State Department needs to approve the pipeline because it crosses national borders. TransCanada claims the pipeline will be the safest in the United States.