By Jeff Mason and Timothy Gardner
WASHINGTON, Feb 24 (Reuters) - President Barack Obama on Tuesday, as promised, swiftly vetoed a Republican bill approving the Keystone XL oil pipeline, leaving the long-debated project in limbo for another indefinite period.
The U.S. Senate Majority Leader Mitch McConnell, after receiving Obama's veto message, immediately countered by announcing the Republican-led chamber would attempt to override it by March 3.
That is unlikely. Despite their majority, Republicans are four votes short of being able to overturn Obama's veto.
They have vowed to attach language approving the pipeline to a spending bill or other legislation later in the year that the president would find difficult to veto.
The TransCanada Corp pipeline would carry 830,000 barrels a day of mostly Canadian oil sands crude to Nebraska en route to refineries and ports along the U.S. Gulf. It has been pending for more than six years.
Obama, who rejected the bill hours after it was sent to the White House, said the measure unwisely bypassed a State Department process that will determine whether the project would be beneficial to the United States.
"Through this bill, the United States Congress attempts to circumvent longstanding and proven processes for determining whether or not building and operating a cross-border pipeline serves the national interest," he wrote in his veto message.
Republicans, who support the project because of its job-creation potential, made passing a bill a top priority after the November election, when they gained control of the U.S. Senate and strengthened their majority in the House of Representatives.
The bill passed by 270-152 in the House earlier this month and cleared the Senate in January.
Obama has played down Keystone XL's ability to create jobs and raised questions about its effects on climate change. Environmentalists, who made up part of the coalition that elected the president in 2008 and 2012, oppose the project because of carbon emissions involved in getting the oil it would carry out of Canadian tar sands.
TransCanada Chief Executive Russ Girling said in a statement the company was "fully committed" to Keystone XL despite Obama's veto and would work with the State Department to answer any questions it has about the project.
Opponents of the pipeline praised Obama's move.
"This veto, along with the president's increasing public skepticism about Keystone XL ... makes us more confident than ever that (the) president will reject the permit itself once and for all," said Gene Karpinski, president of the League of Conservation Voters, another pipeline opponent.
Republicans lambasted Obama.
"The president's veto of the Keystone jobs bill is a national embarrassment," said Republican House Speaker John Boehner. "The president is just too close to environmental extremists to stand up for America's workers. He's too invested in left-fringe politics to do what presidents are called on to do, and that's put the national interest first."
Obama will make a final decision on the project once the State Department finishes its review, expected in the coming weeks.
But the issue is likely to remain central in Washington's political back-and-forth for some time.
The chairman of the House Oversight Committee, Jason Chaffetz, sent a letter to Secretary of State John Kerry on Tuesday asking for all reports and documents received by the State Department from other government agencies about the project, according to an aide. (Additional reporting by Richard Cowan and Susan Cornwell; Editing by Steve Orlofsky)