Environmental activists who have been fighting the Keystone XL pipeline for years were heartened Tuesday by President Barack Obama’s assertion that the emissions impact will be decisive in its fate – but they aren’t ready to declare victory yet.

Weighing in directly on the explosive issue, the president set what appears to be a high bar for approving the project. “The net effects of the pipeline’s impact on our climate will be absolutely critical to determining whether this project is allowed to go forward,” he said during a speech on his administration’s climate change initiatives at Georgetown University in Washington, D.C.

To climate activists who believe Keystone would drive up carbon emissions disastrously, Obama’s pronouncement should lead to the ultimate rejection of the pipeline. But their reaction Tuesday was cautious.

"This is an appropriate standard that the president appears to be setting on Keystone XL,” said Daniel Kessler, campaign manager for the group 350.org, which has been leading organizing against the pipeline. “The president is saying what the science has always demanded. It's encouraging news for certain."

“We think this speech is a clear sign that Keystone XL is going to be held to a very high standard when it comes to its impact on climate change,” said Susan Casey-Lefkowitz, director of the international program at the Natural Resources Defense Council. “I am confident that Keystone XL is a major driver of climate change and so it’s only logical that when all the evidence is in, this is a project that needs to be rejected.”

“We have clear research showing that tar sands oil, which is what goes through the pipeline, has much higher greenhouse gas emissions than conventional oil, but we also have clear evidence that the Keystone XL pipeline would be a major driver of expansion of tar sands,” Casey-Lefkowitz said. Together, she added, “you get a project that is going to significantly worsen climate change.”

“I am optimistic that the Keystone XL tar sands pipeline will be rejected,” said Casey-Lefkowitz, who has led the NRDC’s campaigns to stop the pipeline.

On Capitol Hill, liberals who have been urging Obama to reject the pipeline seemed optimistic as well. “I’m encouraged by the president’s clear and genuine interest in preventing more damage to our atmosphere, and I hope that consideration is foremost in his mind when the final decision is made,” Rep. Raul Grijalva, D-Ariz., said in a statement.

The project, which would carry crude oil from western Canada’s tar sands to refineries on the Gulf Coast, is currently being reviewed by the State Department.