In a tight vote, the Senate rejected the Keystone XL pipeline Tuesday evening, dealing a serious blow to the re-election efforts of Sen. Mary Landrieu (D-La.). (AP)

Senate Democrats blocked a move Tuesday to compel construction of the Keystone XL pipeline, dealing a sharp loss to one of their own, Sen. Mary Landrieu (La.), who had pinned her chances for reelection on approval of the measure.

The vote was a victory for environmental activists who have turned defeat of the pipeline into one of the central symbolic causes of their movement. But Republicans, who will take majority control of the Senate in the next Congress, vowed to return to the fight next year.

On a 59 to 41 roll call, Landrieu’s campaign fell one vote shy of passing legislation meant to force President Obama to approve the nearly 1,700-mile, $7.6 billion project, which would deliver 830,000 barrels of oil a day from western Canada to the American heartland. With just 14 Democrats backing it, Landrieu’s bill fell victim to a filibuster by her own party. All 45 Republicans voted for the measure.

In rejecting the bill, the Senate has granted Obama a temporary reprieve from a difficult decision: whether to side with the environmentalists who have been his staunch allies or with many moderate Democrats who hope to use the issue to win over swing voters.

Already six years in the making, the Keystone fight had become a final rallying cry for Landrieu, a three-term senator facing a runoff election Dec. 6. With her Keystone campaign, she placed a political bet on demonstrating both her clout in Washington and her independence from a very unpopular Obama.

The first thing to know about the Keystone pipeline? It already exists. Here's a breakdown of the pipeline's various parts. (Gillian Brockell, Jhaan Elker and Kate M. Tobey/The Washington Post)

She already faced a steep climb in a conservative state dominated by energy interests, and her task is now even tougher; if she loses next month, Republicans will hold a majority with 54 seats come January, up from their current 45-seat caucus.

“This is for Americans, for an American middle class,” Landrieu pleaded Tuesday evening, moments before the vote, arguing that jobs related to the pipeline would go to rural American communities struggling in the economic recovery. “The time to act is now.”

She then thanked her Democratic colleagues who supported her, including three who lost their elections this month. Once the roll call started, Landrieu stood mostly by herself in the chamber but at one point shared a hug with Sen. Kay Hagan (D-N.C.), one of the defeated incumbents.

Supporters argue that the new pipeline would lead to more efficient delivery of oil into domestic markets, helping secure a reliable source of energy, boosting the national economy and creating jobs tied to the pipeline’s construction. Opponents say it would facilitate the harvesting of oil from the environmentally dirty tar sands in Canada, leading to health risks, and would come online as domestic oil production is already booming.

After the vote Tuesday night, Senate Minority Leader Mitch McConnell (R-Ky.), set to take over as majority leader, told his colleagues that he would bring up the pipeline “very early” next year.

Before the vote, the White House was careful not to issue a veto threat even as officials made it clear that Obama was likely to invoke one should the measure pass the Senate.

“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,” said White House press secretary Josh Earnest. He added that Obama’s senior advisers have recommended vetoes on “similar pieces of legislation” that have been introduced in the past.

After a 59-41 Senate vote rejecting the Keystone XL pipeline was announced on Tuesday, Senate Minority Leader Mitch McConnell (R-Ky.) vowed that the issue will resurface on the agenda of the new Congress in January. (AP)

But, as McConnell made clear, the issue will not disappear. House Speaker John A. Boehner (R-Ohio) has also indicated that he will bring up the matter next year, once Republicans control both chambers. Ten of the Senate Democrats who voted yes will be back next year, adding to the 53 or 54 Republicans whose votes McConnell can count on.

That places the likely support for the pipeline in senatorial limbo — enough to pass a bill and send it to the White House, but a few votes shy of the two-thirds majority needed to override a veto.

That prospect has led some supporters to suggest attaching it to a key spending bill or another must-pass measure, forcing a tougher political choice on the president.

Even some Democrats are open to using approval of Keystone XL as a negotiating chit in exchange for a significant policy concession from congressional Republicans, but it is unclear how receptive White House officials are to that idea.

After the Nov. 4 wipeout for Democrats, Landrieu was thrust into a runoff against Rep. Bill Cassidy (R-La.). State rules require the winner to reach 50 percent of the vote; Landrieu received 42 percent to Cassidy’s 41 percent, as the remaining votes went mostly to other Republicans on the ballot.

With financial backing disappearing in the face of her long odds, Landrieu made passing the Keystone legislation her last-gasp attempt to show voters back home that she still had influence in Washington.

She had run her general-election campaign boasting of her chairmanship of the Energy and Natural Resources Committee, a gavel that she predicted would lead to tangible results for Louisiana. Landrieu has been a strong supporter of the oil and gas industry, well before and beyond the Keystone XL fight.

In the wake of the 2010 oil spill in the Gulf of Mexico from BP’s Macondo well, Landrieu pushed the Obama administration to lift its moratorium on drilling in the gulf. She personally lobbied Interior Secretary Ken Salazar and put a hold on the nomination of Jack Lew to head the Office of Management and Budget, angering Obama.

“I would not be alone in telling you that Senator Landrieu is one of most influential and effective advocates for the industry that we have on the Hill regardless of party affiliation,” said Jim Noe, senior vice president and general counsel of Hercules Offshore, which provides marine support to offshore drillers.

Senate Majority Leader Harry M. Reid (D-Nev.), after years of tangling the chamber in knots when it came to the pipeline, relented to Landrieu last week and allowed Tuesday’s debate and vote, even though he remained opposed to the measure and advised Obama to veto it.

Under a bipartisan agreement, Landrieu was given a single vote on the legislation with a supermajority threshold of 60 votes. Last week, House Republican leaders allowed Cassidy to author and pass an identical measure, making Landrieu’s defeat more politically painful Tuesday.

Landrieu had predicted victory Monday night, telling reporters that she felt “very comfortable” with her ability to hit the magic number.

So as debate began Tuesday, Landrieu’s biggest opponents — liberals such as Sen. Barbara Boxer (D-Calif.), chairman of the Environment and Public Works Committee — were also her biggest supporters.

“Let the record be clear forever: This debate would not be before this body were it not for Sen. Landrieu’s insistence,” Boxer said in her opening remarks Tuesday morning.

Hours later, Boxer reiterated her praise: “Without Mary Landrieu we would not be having this debate.”

Steve Mufson and Ed O’Keefe contributed to this report.