(Adds reaction from Canadian prime minister and TransCanada)
By Timothy Gardner and Richard Cowan
WASHINGTON, Jan 6 (Reuters) - Republican senators kicked off the new U.S. Congress with legislation to approve the Keystone XL pipeline to bring oil from Canada, but the White House promptly threatened a veto.
With Republicans assuming full control of Congress on Tuesday after victories in the November elections, they have put Keystone at the center of their agenda and plan weeks of debate.
They believe that the public spotlight on Keystone will pressure President Barack Obama to eventually approve the project.
The White House was adamant that Obama would not sign the Keystone bill.
"There is already a well-established process in place to consider whether or not infrastructure projects like this are in the best interest of the country," White House spokesman Josh Earnest told reporters.
Senator John Hoeven, a North Dakota Republican, introduced a bill to approve TransCanada Corp's project that would transport more than 800,000 barrels per day of mostly Canadian heavy oil to Nebraska en route to refineries along the U.S. Gulf Coast.
Obama, who has been considering the pipeline for six years, has opposed previous Keystone bills, saying the State Department needs to complete its approval process.
Both Republican and Democratic lawmakers predicted that 63 senators would back the pipeline, enough for the bill to pass but short of the 67 that would be needed to overcome a veto.
If Obama vetoes the bill, backers will attach it to a wider measure he could find harder to reject, such as a must-pass spending legislation or steps to improve energy efficiency.
"We may not have enough to overcome a veto, so it may be a two-step process," Hoeven told Reuters.
New Senate Majority Leader Mitch McConnell has promised to open the amendment process on legislation, allowing the full Senate to weigh in on proposed additions to bills.
But Democrats accused Republicans of trying to rush the measure to the floor after McConnell moved to bypass the energy committee which was scheduled to debate the bill this week. It was uncertain whether the energy committee would debate the bill on Thursday or if it would go straight to the Senate floor.
Keystone has divided Democrats between environmentalists, who say oil sands mining will raise emissions linked to climate change, and union supporters who say it will add thousands of construction jobs and boost energy security.
Canadian Prime Minister Stephen Harper repeated his view, through a spokesman, that Keystone should be approved, saying it would create jobs in both countries.
The head of TransCanada complained about the veto threat, saying the Obama administration's review process seemed to have no end. "The bar continues to move again and again," said Chief Executive Russ Girling.
Reporting by Timothy Gardner and Richard Cowan, additional reporting by Jeff Mason, Randall Palmer and David Ljunggren in Ottawa, and Scott Haggett in Calgary; Editing by Marguerita Choy and Caren Bohan