President Obama used his veto power on Tuesday for just the third time in his administration, sending back to Congress legislation approving construction of the Keystone XL pipeline, which would transport 800,000 barrels of oil per day from Alberta, Canada to refineries along the Gulf Coast.

"The Presidential power to veto legislation is one I take seriously. But I also take seriously my responsibility to the American people," Obama wrote in a two-paragraph communication to Congress. "And because this act of Congress conflicts with established executive branch procedures and cuts short thorough consideration of issues that could bear on our national interest — including our security, safety, and environment — it has earned my veto."

Obama has long pledged to send back to Congress the Keystone bill while the State Department continues to evaluate the merits of the project, a process that is now in its sixth year. The administration's lengthy review has drawn the ire of Republicans and some Democrats in Congress.

Senate Majority Leader Mitch McConnell pledged that the Senate would "soon vote on an override."

Neither the Senate nor the House of Representatives can currently muster the two-thirds majority necessary for overturning the President's veto. The Senate remains four votes short, the House remains 11 votes shy.

In a video posted to his Twitter account, House Speaker John Boehner said the fight to approve Keystone was far from over.

"You know, the President says he's for the middle class, and he's for more jobs, but then he turns around and vetoes the Keystone pipeline," he said. "You have to wonder: What's he thinking?"

Environmentalists quickly hailed the decision, but they urged the administration to issue a final ruling that the project would exacerbate the impacts of climate change and is therefore not in the national interest.

"We're glad the president vetoed this cynical, politically-motivated stunt," Amanda Starbuck of the Rainforest Action Network said. "However, a veto on its own is not enough. This movement has fought for years to stop the Keystone XL pipeline and we will accept nothing less than a final rejection. Keystone XL miserably fails the climate test, and if President Obama wants to be remembered as a leader on climate, his only option is rejection."

I may just watch this clip of the official veto of — David Turnbull (@david_turnbull)February 24, 2015

International diplomats have set a December deadline for securing an international climate agreement aimed at reducing global greenhouse gas emissions. Against this backdrop, climate activists have emphasized the hypocrisy of pledging to confront global warming, while enabling expanded fossil fuel use, should Obama approve the bill.

"Signing the Keystone XL legislation would have sent the wrong signal to international leaders looking to deliver a new global climate pact in Paris later this year, and undercut our nation's ability to drive progress toward that goal," said Todd Shelton, vice president for US government relations at the World Wildlife Fund. "The US must continue to exert strong leadership in charting a course to a future powered by clean, renewable energy rather than dirty and dangerous fossil fuels."

But just as environmentalists began to celebrate outside the the White House, administration spokesperson Josh Earnest ensured continued speculation over the ultimate fate of the pipeline.

When asked if the Obama administration might approve the pipeline following its executive branch review, Earnest said: "That possibility still does exist."