WASHINGTON - After six days of political wrangling and vote-whipping, the Senate failed to pass a bill on Tuesday forcing authorization of the Keystone XL pipeline, dashing hopes of Sen. Mary Landrieu (D-La.) to add the vote to her list of accomplishments heading into a tough runoff election.

Fifty-nine senators voted for the bill, one short of the 60 needed to clear a filibuster. Fourteen Democrats joined all the Senate Republicans in voting for the bill, which was cosponsored by Landrieu and John Hoeven (R-N.D.). The House passed companion legislation on Friday from Rep. Bill Cassidy (R-La.), Landrieu's opponent in the runoff election.

Landrieu said going into Tuesday's debate that it was "one of the first debates I've been in in eight years where the outcome is uncertain." She added, however, that she went into the debate "knowing in my heart we have 60 votes. I hope we've got the courage that supports that."

But that last vote never materialized.

At a press conference after the vote, Landrieu reflected on the loss but did not blame fellow Democrats who voted against her bill. "There is no blame," said Landrieu. "There is only joy in the fight."

"I arrived with 14 votes and felt relatively certain that the coalition we put together was strong enough to find the extra one," she continued. "I'd say we have to work that muscle a little more."

Landrieu also expressed hope she'd be back in 2015. "I'm going to fight for the people of my state until the day I leave," said Landrieu. "I hope that will not be soon."

Landrieu touted the vote as an example of her experience in the Senate. "Only a senior member who has been here as long as I have could recognize the opportunity when Mitch McConnell was focused on other issues and Harry Reid was going back to the same old agenda" she said. "I said you know what, I think it's time to have this debate. My experience is still valuable, my chairmanship of the Energy Committee until the end of this year is very valuable to the people of the state of Louisiana."

Democrats who backed her fight also voiced support. "I was proud of Mary, the fight she led for us," said Sen. Joe Manchin (D-W.Va.) "Mary got that debate for us. This day would have never come without Mary leading the charge."

Several Democrats considered swing votes confirmed in the days leading up to the vote that they would be voting against the bill. And on Tuesday afternoon, Angus King, a Maine independent who caucuses with the Democrats, announced that he was voting "no" on the measure. King had previously said he was "leaning no." His was among the last uncertain votes in the Senate.

"Congress is not -- nor should it be -- in the business of legislating the approval or disapproval of a construction project," King said in a statement Tuesday afternoon. "And while I am frustrated that the President has refused to make a decision on the future of the pipeline, I dont believe that short-circuiting the process to circumvent his Administration is in the best interest of the American people."

Environmental advocates tried to put pressure on a pair of Democrats who had decided to vote for the bill, Sens. Tom Carper (Del.) and Michael Bennet (Colo.). Capitol Police arrested four young activists who staged a protest in Carper's office, singing refrains like "Stop the pipeline, stop the greed." Three others were arrested at Bennet's office.

Debate on the bill lasted six hours, with supporters of the bill criticizing the evaluation process that has now lasted six years. "This process has not worked," said Sen. Heidi Heitkamp (D-N.D.). "This process has not brought this project to some kind of finality."

Critics of the pipeline took to the floor to defend their votes against the bill. "From my state's point of view, it's all harm," said Sen. Sheldon Whitehouse (D-R.I.).

The failure averts a showdown with President Barack Obama , for now. While the president never issued a direct veto threat, he strongly implied in a press conference Friday that he would reject the bill if it came to his desk. "My position hasnt changed, that this is a process that is supposed to be followed," he said.

But legislation to force approval of the pipeline will likely make a comeback as soon as the new Congress is sworn in in January, when a bill will have more supporters in the Senate. "Republicans are committed to getting Keystone approved," said Minority Leader Mitch McConnell (R-Ky.) on the Senate floor. If not today, McConnell said, "then a new majority after the beginning of the year will be taking this up and sending it down to the desk of the president."

After the bill failed, co-sponsor Hoeven said he thinks there's a strong chance of passing the legislation with a veto-proof majority next year, or including it in a broader package.

The Senate is expected to have 63 votes in favor of approving a bill like Landrieu's come January. That's enough to clear the filibuster, but not a veto-proof majority. However, Republicans hope to pick up additional Democratic votes for the measure if it is included in a larger package, or if some Democrats who voted against it on Tuesday are more inclined to vote for it after a decision on the route through Nebraska.

"We'll have a strategy and we'll come back, and I really am confident that we'll not only get it, but that we'll get it in a way that gets us beyond a presidential veto," Hoeven said.

Russ Girling, president of TransCanada, the company seeking permission to build the Keystone XL, said in a statement that the vote "demonstrates a growing and high level of support" for the pipeline. "Senators Mary Landrieu and John Hoeven are to be commended for leading a bipartisan coalition in support of a legislative solution to the protracted regulatory process Keystone XL has languished in for six years," said Girling.

This article has been updated with additional comments from senators and from TransCanada.

CORRECTION: This article has been edited to correct Sen. Heidi Heitkamp's state, North Dakota.

Elise Foley contributed reporting.