Senate Democrats filibustered the Keystone XL pipeline on Tuesday, in a vote that reverberated from Louisiana, where a key senator’s career is now likely doomed, to the broader national Democratic Party, where environmentalists have emerged triumphant in a divisive internal battle with labor unions.

The Keystone vote took on symbolism far beyond the small impact on American crude supplies and the slight effect expected on gas prices. Environmentalists drew lines and dared moderate and conservative Democrats to cross it. In the end, most were unwilling to defy the ascendant movement, and it marked a key moment in the climate change debate.

“It’s time to make this turn. And there’s no better moment to make this turn than on this pipeline that would bring the filthiest fuel in the planet into circulation,” said Sen. Sheldon Whitehouse, a Rhode Island Democrat.

The vote fell one shy of the 60 needed to overcome the filibuster, with 14 Democrats joining all 45 Republicans in backing the project.

In the near term, the vote means President Obama gets to avoid a difficult decision on whether to veto the bill. But Senate Republicans said he shouldn’t get too comfortable because they will pass a new version as soon as they take control of the chamber next year.

Based on Tuesday’s vote and Senate balance next year, Republicans should have enough support to overcome a Democratic filibuster but not enough to override a presidential veto.

Mr. Obama refused to say whether he would veto this year’s bill, though he sounded skeptical when he spoke to reporters over the weekend. He said Congress should give him leeway to make a decision on his own.

Traditional blue-collar labor unions, though, desperately sought the pipeline’s approval, saying it was a test of whether the Democratic Party could be trusted on jobs.

“The majority of Democrats in the Senate and the White House just don’t get it, even though the recent election results surely should have sunk in by now. They have lost their way, their purpose and their base,” said Laborers’ International Union of North America President Terry O’Sullivan.

State Department officials have been delaying a decision for years, trying to decide whether to approve the final permit that would allow the pipeline to cross the international boundary between Canada and the U.S., bringing crude from the oil deposits in the tar sands of Alberta to refineries in the U.S.

The pipeline is also awaiting action by courts and permitters in Nebraska, where some residents have challenged its proposed path. They say the pipeline would threaten critical natural habitat.

The Senate vote proved to be dramatic, chiefly because it wasn’t clear what the outcome would be.

Vote counters said over the weekend that Sen. Mary L. Landrieu, the Louisiana Democrat who pushed for the vote, was just shy of the support she needed — though she was optimistic.

Her re-election bid is now likely doomed. Ms. Landrieu was already struggling to unify her party and eat into Republican support, and the pipeline, which she set up as a test of her leadership, backfired.

Indeed, the comparison with her Republican opponent, Rep. Bill Cassidy, is striking. Mr. Cassidy was the sponsor of the House version of the Keystone bill, which cleared the lower chamber last week in a bipartisan vote.

Her fellow Democrats did their best to defend Ms. Landrieu during the floor debate, praising her efforts even as they prepared to filibuster her bill and defeat her chief case for re-election.

“Without Mary Landrieu, we would not be having this debate,” said Sen. Barbara Boxer, chairwoman of the Senate Environment and Public Works Committee, who led opposition to her fellow Democrat.

An emotional Ms. Landrieu stood in the well and watched as her colleagues voted and likely sealed her fate back home.

Afterward, she told reporters she resented both sides trying to play politics with the issue and said she believed she had forced the issue onto the agenda thanks to her seniority in the Senate.

“There’s no blame. There’s only joy in the fight,” Ms. Landrieu said.

Analysts said the vote was everything from a statement on U.S.-Canada relations to a proxy for where Congress stands on climate change and the future of fossil fuels in the economy. Some also said it hinted at a battle within Senate Democrats’ ranks over whom they wanted to be the next top Democrat on the Energy and Natural Resources Committee and, more broadly, over which wing will control the Democratic Party going forward.

“The totemic nature of it can’t be avoided,” said Michael McKenna, a Republican energy lobbyist. “It’s essentially a choice between Mary Landrieu and Maria Cantwell, between [Senate Majority Leader] Harry Reid and [West Virginia’s Sen.] Joe Manchin, if you want to think of it that way. Between a party that used to be a fairly reliable oil and gas party, the Democrats, and what they want to be in the future — a party in favor of exotic and expensive energy.”

Jane Kleeb, director of Bold Nebraska, one of the major groups opposing the pipeline, said the vote was a challenge to Democrats.

“The Democratic Party must lead on energy in order to not only win elections but to protect our land and water,” she said. “There is no place in the party to keep on the outdated mantra of drilling more. We’ve done that for 100 years, it’s time to diversify our energy base. Sen. Manchin can represent coal jobs and families. He just can’t do that at the expense of others. That type of horse trading has to be over.”

After the vote, a dozen environmental groups issued an ebullient statement saying they had won the fight and expressed optimism that Mr. Obama’s State Department would ultimately reject the pipeline.

“We’re more confident than ever that this pipeline will never be built,” said League of Conservation Voters Senior Vice President Tiernan Sittenfeld.

The bill would have removed Mr. Obama’s final say and officially approved the pipeline under terms submitted to the State Department.

Supporters and opponents could agree on little. Ms. Boxer warned of the danger of environmental catastrophe from spills and said the oil wouldn’t end up in the U.S. but instead would be shipped from refineries on the Gulf of Mexico to foreign ports.

“I call it the Keystone extra lethal pipeline,” she said in front of a poster of a child wearing an oxygen mask.

Keystone defenders predicted the oil would end up in the U.S. and be shipped to ports in states such as Florida. They also said the drills would go no matter what — the question is whether the oil is shipped to the U.S. or China, and if it comes to the U.S., whether it’s shipped by dangerous railway cars or by what they said was the most advanced pipeline ever built.

“The pipeline has taken a role in American politics that is way disproportional to what it is,” said Sen. Heidi Heitkamp, a North Dakota Democrat whose state has undergone an economic renaissance based on drilling. “It is a pipeline. There are over 2 million miles of pipelines in America today. This is going to be just another one of those, and it’s going to be state of the art.”

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