A memo sent to the White House by lead environmental groups argues the Keystone XL pipeline planned from Canada fails key climate tests.

Pipeline company TransCanada submitted the application to the U.S. federal government necessary to build Keystone XL across the U.S.-Canadian border more than six years ago. The project has faced stiff opposition from climate groups worried about the effects the heavier grade of Canadian crude oil would have on the environment.

A memo sent to President Barack Obama from seven environmental groups, led by the Natural Resources Defense Council and Oil Change International, argues the pipeline would add to the development of Canadian crude oil and subsequently add millions of tons of carbon pollution to the atmosphere.

"Market forces and the latest climate science make it clear Keystone XL shouldn't be approved because it would drive up carbon pollution, making climate change worse and jeopardizing our future," Anthony Swift, staff attorney in NRDC's international program, said in the memo sent Thursday. "It's not a plan to help our country."

The environmental groups point to Obama's argument last year that Keystone XL would only serve the nation's interest if it "does not significantly exacerbate the problem of carbon pollution." Last month, they note, a U.N. climate panel found fossil fuels are contributing to global warming.

A State Department review of the pipeline found some of the environmental issues cited by critics would be present with or without Keystone XL.

Supporters of the project argue Keystone XL would be a regional economic stimulus and an anchor for energy security. Republican leaders, who in January take control of both chambers of the U.S. Congress, said getting the project approved will be their first order of business.

The narrative from both sides of the debate has been fluid. Oil Change International and its counterparts argue in their latest memo the current low price of crude oil should be considered as a strike against the economics behind the project. In 2011, the criticism was targeted at conflicts of interest in Washington, where a former campaign manager for Hillary Clinton, Paul Elliot, went to work as a lobbyist for TransCanada.

For backers, the project was billed initially as a way to expand deliveries of crude oil from Canada, the top exporter of crude oil to the United States. Supporters have said as recently as this week the project would help ferry oil taken from shale deposits in North Dakota.