NRDC attorney Danielle Droitsch described Keystone in a statement Thursday as a "dirty energy project that will drive tar sands development."
"Every day we're seeing more evidence why the Keystone XL tar sands pipeline fails that test and should be denied," she said.
NRDC Clean Air Policy Director David Doniger said the advocacy group would "be focused like a laser beam" on ensuring federal regulators work to curb methane emissions associated with oil and natural gas development.
NRDC officials say the organization is looking ahead to 2014 as a year of dramatic efforts to address climate change.
Pro-renewable energy groups like the NRDC say oil production, particularly from heavier grades like tar sands, may lead to higher emissions of greenhouse gases.
Supporters of Keystone XL say it will provide a source of economic stimulus and energy security.
It needs approval from the federal U.S. government as a cross-border pipeline.
IHS Cambridge Energy Research Associates said in a November report the greenhouse gas intensity of the heavier type of Canadian crude oil designated for Keystone XL is on par with other sources of crude oil imported into the United States.