Forty Democrats and Angus King, independent of Maine, voted against the bill, with just 14 Democrats joining all 45 Republicans in support of the oil pipeline.
The battle over approving the pipeline, which will carry petroleum from the oil sands of Canada to the Gulf Coast of Texas, ultimately became a proxy war for the Louisiana Senate seat, where Ms. Landrieu and Representative Bill Cassidy, a Republican, are locked in fight for votes in their oil-rich state ahead of a Dec. 6 runoff election.
Ms. Landrieu — who, if re-elected, will lose her coveted position as chairwoman of the Energy Committee when Republicans take the Senate majority next year — spent the past few days working furiously to round up Democratic support for her bill, which she had hoped would be her last, best chance of holding on to her Senate seat.
On Tuesday morning, she was at least one vote short of the filibuster-proof 60 votes she needed. And despite cajoling, persuading, browbeating, and making an impassioned plea to her colleagues during a closed-door lunch — which one attendee described as “civilized but pretty contentious” — Ms. Landrieu, who has so often bulldozed her way to success through sheer force of will, came up just short.
The House, which passed the same legislation on Friday, had voted multiple times already to approve the pipeline. But Tuesday’s vote marked the first time this year that the Senate majority leader, Harry Reid of Nevada, agreed to hold on a vote on the bill, which he feared could have hurt the re-election changes of some of his more vulnerable members.
Both Mr. Cassidy and Ms. Landrieu were eager to take credit for supporting the Keystone bill back home, where their state’s economy is heavily dependent on oil-industry jobs. Speaking on the floor, Republicans sought to cast the legislation as “Congressman Cassidy’s Keystone jobs bill,” while Democrats described it as Ms. Landrieu’s brainchild.
Even Senator Barbara Boxer, Democrat of California, who did not support the bill and said Keystone XL stood for “extra lethal,” was sure to note that credit for the legislation belonged to Ms. Landrieu.
However, even had the Senate passed the bill, Mr. Obama was not expected to sign it into law.
Before the vote, White House aides stopped short of an explicit veto threat, but left the impression that the president would reject the bill if it made it to his desk.
Josh Earnest, the White House press secretary, said, “It certainly is a piece of legislation that the president doesn’t support, because the president believes that this is something that should be determined through the State Department and the regular process that is in place to evaluate projects like this.”
The administration has said previously that it does not plan to offer a final decision until after a Nebraska court weighs in on the route of the pipeline through that state, a decision that could come as early as January. By then, Republicans will control both the House and the Senate, and are expected to send the bill to the president’s desk, where Mr. Obama might be more willing to use it as leverage to extract other concessions from a Republican-controlled Congress.
Most Americans support the Keystone pipeline — a Washington Post-ABC News poll in May found that 65 percent of Americans think the pipeline should be built, while just 22 percent oppose it.