"The Keystone XL pipeline will supply both energy from our closest friend and partner and create jobs in the United States," the letter reads. "Further delay will continue to hurt job creation and may damage our relationship with Canada."
Pipeline company TransCanada aims to build the pipeline to get more oil from Alberta to refineries along the southern U.S. coast. Supporters of the project say it will provide an economic stimulus while ensuring North American energy independence. Detractors say the environmental risks of tar sands oil aren't worth the rewards.
Oil Change International, a group opposing the project, said lawmakers who signed the letter received a total of $13.6 million in support from energy companies since 1999.
"No matter how you look at it, it's clear whose interests these senators are looking out for," a statement reads.
The U.S. government needs to sign off on Keystone XL because it would cross the U.S.-Canadian border.