Maine Sen. Angus King joined most Senate Democrats on Tuesday in blocking a bill to allow construction of the Keystone XL pipeline, casting a pivotal vote on a bill with implications for the climate change debate and one Democratic senator’s political future.

King, an independent who caucuses with the Democrats, has been courted as the possible 60th vote needed to advance a bill to bypass the Obama administration’s review of the controversial pipeline project. But the bill’s failure Tuesday on a 59-41 vote could also cost Sen. Mary Landrieu, a Louisiana Democrat facing a tough runoff election in a state where oil refining is big business.

Maine Sen. Susan Collins, a Republican, voted as expected to allow construction to proceed. But King, after being in the national spotlight for several days, ended speculation Tuesday morning hours before the vote even as he expressed frustration with the slow pace of the State Department review.

“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. “And while I am frustrated that the president has refused to make a decision on the future of the pipeline, I don’t believe that short-circuiting the process to circumvent his administration is in the best interest of the American people. I urge the president to make a decision soon, and, if he doesn’t, I look forward to working with Congress to put a time frame on this decision.”

As proposed, TransCanada’s proposed Keystone XL pipeline would have the capacity to carry up to 830,000 barrels “oil sands” or “tar sands” crude oil from the Canadian province of Alberta to Nebraska. Once in the U.S. heartland, the Alberta crude oil would flow onto other pipelines to be carried to refineries along the Gulf Coast.

Supporters argue the project will create thousands of construction jobs and reduce U.S. reliance on oil from more turbulent regions. Opponents question those job figures and denounce “tar sands” oil as environmentally irresponsible.

TransCanada was required to receive State Department approval for the 1,200-mile section of the pipeline because it crosses the U.S.-Canada border. But the Obama administration, facing intense opposition from well-funded environmental groups and some landowners, has delayed any decision.

Tuesday’s vote highlighted the political dynamics of the pipeline debate, however.

Facing a runoff election against a Republican opponent, Landrieu has waged a personal campaign to pass a bill that would benefit her state’s refining industry but is opposed by most members of her party. Landrieu and her supporters hoped Senate passage would help narrow the lead enjoyed by her opponent, Republican U.S. Rep. Bill Cassidy, and keep one more seat in Democratic hands when Republicans take control of the Senate next year.

The Associated Press reported that bill supporter Sen. Joe Manchin, D-W. Va., said Landrieu appealed to her fellow Democrats during a closed-door lunch, saying “this vote is going to happen, whether it happens now or it happens in January, same outcome, so why not do it now?”

All 45 Senate Republicans – including Maine Sen. Susan Collins – were expected to vote for the bill. Without King’s support, Landrieu appeared stuck at 59 votes – one short of the 60 needed to advance the measure under the Senate’s complex procedural rules.

Collins said prior to the vote that it is “past time for the president to have made a decision” and added that blocking the pipeline will push the oil onto ships or railroad cars, creating other safety concerns.

“Canada has made clear it will develop this resource,” Collins said in a statement. “I would much rather see our nation receive and refine the oil from our close ally than have it shipped overseas to Asia. Canada is our nation’s largest trading partner, and this pipeline, constructed at the highest safety standards, would create thousands of jobs in our two nations—both in building the pipeline and in the refining of crude oil, one reason why many labor unions support its construction.”

Republican leaders, meanwhile, vowed to continue fighting for the bill next year. House Republicans passed a similar measure last week that was sponsored by Cassidy, Landrieu’s opponent.

“I wish the Senate would have followed the lead of Congressman Cassidy and his House colleagues in approving Keystone years ago. It’s just common sense,” Sen. Mitch McConnell, the Kentucky Republican slated to become the next Senate majority leader, said prior to the vote. “And if not, a new majority will be taking this matter up and sending it to the president.”

White House spokesman Josh Earnest said Tuesday that Obama does not support the Senate bill in the clearest indication yet that the president could veto any legislation, The Associated Press reported.

Both of Maine’s two senators are frequently courted as potential swing votes on closely divided issues, although the two appear to be on opposite sides of the Keystone issue.

In 2013, King opposed a non-binding budget amendment urging approval of the Keystone pipeline. He is a vocal critic of Republican resistance to addressing climate change – a focal point of pipeline opponents – because of the implications for Maine’s commercial fisheries and other natural resources industries. And the independent has called for an intensive federal environmental review of long-rumored proposals to move tar sands oil through an existing pipeline between Montreal and South Portland.

With the fate of the bill still hanging in the balance, King has been the target of pressure campaigns from both sides in the Keystone debate.

On the eve of the Senate vote, the American Petroleum Institute apparently sponsored automated telephone calls, or “robocalls,” to Maine voters saying, “it is time for the Obama Administration and Congress to stop putting political interests ahead of what’s best for the nation.”

“Please call Senator Angus King . . . and tell him to do the right thing and support the Keystone XL pipeline,” the robocall stated, according to a recording of the call. An American Petroleum Institute spokesman declined to comment specifically on the calls but said the industry trade group as “a broad ongoing national issue education campaign that is focused on the benefits of energy development, jobs and energy security, including benefits of approving the Keystone XL pipeline.”

Opponents of the pipeline have also been lobbying King, a former two-term governor who was heavily involved in development of wind power and other renewable energy in Maine.

Glen Brand, director of the Maine chapter of the Sierra Club, praised King’s decision to vote against the Senate bill.

“By saying ‘no’ to Keystone and dirty tar sands oil, Senator King is demonstrating much-needed leadership on climate [change] and environmental protection,” Brand said Tuesday afternoon.

Arguably the nation’s most hotly contested environmental issue, the Keystone XL pipeline proposal encapsulates the jobs vs. environment debate and political tensions over addressing emissions of climate-warming greenhouse gases.

Supporters contend the pipeline will create tens of thousands of new construction jobs and help reduce U.S. reliance on oil from the Middle East and other oil-rich regions not as friendly to American interests as Canada. They also claim that Alberta’s petroleum deposits – referred to as “oil sands” by the industry’s supporters and as “tar sands” by opponents – will be extracted from the ground. The only question is whether that crude will be shipped by pipeline or by rail car.

Keystone’s opponents claim that extraction of tar sands oil causes more on-the-ground environmental destruction and generates more greenhouse gases. They also contend that construction of the pipeline will trade short-term construction jobs for the long-term implications of continued reliance on fossil fuels at a time when the U.S. should be investing heavily in renewable energy.

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