The US Senate braced Tuesday for its most contentious vote since midterm elections -- approval of the controversial Keystone XL oil pipeline, a bill President Barack Obama has signalled he might veto.
The vote itself will be a nailbiter. Under special rules for the measure, 60 votes are needed for passage in the 100-member Senate, and supporters acknowledged early Tuesday they remained one vote short.
"We know we've got 59. We think we maybe have 60, but we won't know until the vote," said Senator John Hoeven, the bill's Republican co-sponsor.
He said he has been on the phone "non-stop" seeking to convince colleagues ahead of the 2315 GMT vote.
Congressional Republicans have long pushed the Obama administration to lift its hold on the corporate-funded, $5.3-billion project, which remains under extended review some six years after permit requests were submitted.
The pipeline, which would bring oil from tar sands in Alberta, Canada to refineries on the US Gulf coast, easily passed the Republican-controlled House last week, as it has several times before.
- Democrats clear way for vote -
With Senate Majority Leader Harry Reid now allowing a vote on the bill, Tuesday's action in the Senate -- where leadership shifts from Democrats to Republicans in January as a result of the November 4 elections -- will mark the closest Keystone has been to approval.
Democratic Senator Mary Landrieu, a co-sponsor of the bill, expressed optimism on the floor, but acknowledged she would be intensively lobbying fellow Democrats to get the elusive 60th vote needed to win.
The chamber's 45 Republicans are on record supporting Keystone, as are 14 Democrats.
"I brought this bill to the floor knowing in my heart that we have 60 votes. I sure hope we've got the courage that supports that," Landrieu said.
Heightening suspense, independent Senator Angus King, seen as a potential decider, declined to say Monday how he will vote.
Keystone's progress is closely monitored in Canada, where government officials and builder TransCanada have said the project would provide an economic boon.
But Obama hinted he may block the bill should it reach his desk.
"We're going to let the process play itself out," the president said Sunday in Australia, referring to a State Department review of the project and an ongoing Keystone-related court case in Nebraska.
"But I won't hide my opinion about this, which is that one major determinant of whether we should approve a pipeline shipping Canadian oil to world markets, not to the United States, is does it contribute to the greenhouse gases that are causing climate change."
The State Department, which has authority over Keystone because of its international link, released a review in January finding that the project's carbon emissions would not be significant.
- Jobs versus environment -
Critics warn of the pipeline's environmental impact, arguing that tar-sand bitumen is some of the world's dirtiest oil.
Standing before a poster reading "Misery follows tar-sands," Senate Democrat Barbara Boxer rattled off a list of health concerns, including asthma and heart disease, exacerbated by such oil extraction and refinement.
"Time out for a minute here," Boxer told colleagues. "What are we doing to our people to say that we're helping with the tar sands?"
Senate Minority Leader Mitch McConnell said Keystone opposition was merely political nitpicking.
"The whole drama over Keystone has been as protracted as it is unnecessary," he said.
Republicans tout Keystone's generation of 40,000 temporary construction jobs. The State Department agrees, but says only 35 permanent jobs will result.
Republicans gained at least seven Senate seats in the midterms, and when the new Congress convenes with McConnell at the helm they should have votes to pass Keystone.
But Landrieu forced a vote because she is locked in a December 6 runoff in Louisiana, where she wants to be seen as breaking Washington gridlock.
Her Republican challenger, congressman Bill Cassidy, introduced the parallel legislation that passed in the House.