But Republican and Democratic supporters are already vowing to make the pipeline project a priority during the next Congress, which convenes in January.

Sen. Lisa Murkowski (R-Alaska) said shortly after the vote, “I’m disappointed that we couldn’t get the votes tonight to approve this important infrastructure project, but this isn’t over."

Murkowski, who is expected to chair the Senate Energy and Natural Resources Committee in the 114th Congress, said, "The Keystone XL pipeline will be a top priority next year for the new majority.”

A total of 14 Senate Democrats and all 45 Republicans voted to approve the pipeline. The House passed its version of the bill last week. The Senate vote was a major setback for the measure's prime advocate, Mary Landrieu (D-La.), whose re-election bid hinges on a runoff with Rep. Bill Cassidy (R-La.).

The 1,179-mile project would run from the Tar Sands regions of Hardisty, Alberta, to Steel City, Neb. The State Dept. estimated the cost at $3.3 billion.

TransCanada filed an application in 2012 with the State Dept. seeking approval for the current configuration of the project. The department's review was needed ecause the project would cross the U.S. border.

In January, the State Dept. released its final environmental impact statement on the project. It said its analyses "suggest that there would be no significant impacts to most resources along the proposed project route," assuming TransCanada adheres to current laws and regulations and carries out its proposed mitigation measures.

A period for public and federal interagency comments followed.

Meanwhile, Nebraska's Supreme Court is currently considering a challenge to the TransCanada project's route through the state. The project’s opponents said Republican Gov. David Heineman lacked the authority to make a decision to approve a modification to the route, a decision usually handled by the state Public Service Commission.

The White House hasn't issued a veto warning regarding the current Keystone bill.

President Obama, who is expected to make the administration's final decision on whether to approve the project, said in a Nov. 16 press conference that "we're going to let the [Keystone review] process play itself out."

He added, "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 greenhouse gases that are causing climate change."

The project’s fate remains unclear and there is some speculation that the president could ultimately approve the pipeline.

Environmental groups oppose the project, which they say could accelerate climate change and potentially pollute drinking water supplies.

Anna Aurilio, Environment America global warming solutions program director, said in a statement, “Unfortunately, we know this isn’t the last time the Senate will attempt to do the bidding of Big Oil. Ultimately, we’re counting on the President to continue his leadership on climate and veto any measure to force approval of this dirty pipeline.”

But labor unions, which view the project as a job creator, blasted the Democratic lawmakers who did not vote in favor of moving forward with the pipeline.

Terry O’Sullivan, Laborers' International Union of North America general president, said, “That Democrats would block the pipeline’s approval in spite of the fact that repeated environmental impact statements have all concluded that the Keystone XL will have no appreciable impact on greenhouse gas emissions, leaves us disillusioned, disgusted and exasperated with the current majority party and it leaves no doubt as to why they will soon be the minority party. 

He added, "Continued pandering to environmental extremists, who want to hamper the American economy and destroy jobs is a recipe for continued election losses.”