Environmentalists are claiming that approving the Keystone XL pipeline will harm consumers by raising prices at the pump.

A report by Consumer Watchdog, a left-wing group, found that the Keystone pipeline would drive up U.S. gas prices — 20 to 40 cents in the Midwest — and won’t yield any long-term benefits for the U.S. economy.

The group claims the companies that own the pipeline want to use it to “reach export outlets outside the U.S. for tar sands oil and refined fuels, which would drive up the oil’s price.”

“A vote for Keystone is a vote to raise gas prices on Americans and send the profits to a foreign oil company,” said former hedge fund manager and anti-Keystone activist Thomas Steyer, who backed the report.

However, Keystone supporters disputed the claims that gas prices would rise.

“Any analysis which attempts to show that adding 830,000 barrels per day of highly-discounted oil to U.S. markets will raise gasoline prices is not worth the paper it is written on,” said Michael Whatley, vice president of the Consumer Energy Alliance. “Clearly, Consumer Watchdog and its anti-Keystone XL billionaire backer, Tom Steyer, continue to choose ideology over logic.”

Aside from possibly raising gas prices, environmentalists, including Steyer, have opposed the approval of the Keystone pipeline over concerns that it will contribute to global warming and harm the environment.