WASHINGTON — Republican leaders promised Tuesday to take up and pass a bill approving the Keystone XL pipeline next year if the Senate fails to advance the measure this fall, or President Barack Obama vetoes it.

Senate Minority Leader Mitch McConnell, who will become the majority leader in January, urged Democrats to vote Tuesday evening for the bill, which is supported by all 45 Senate Republicans. Sen. Mary Landrieu, D-La., was still searching for the last vote needed to advance the measure and said on the floor she knew “in her heart” she had the 60 votes.

The House approved a bill last week spearheaded by her rival, Louisiana Republican Rep. Bill Cassidy, the ninth time that chamber has tried to jumpstart the pipeline’s construction. Landrieu faces an uphill fight to hold on to her Senate seat in a Dec. 6 runoff against Cassidy.

“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,” McConnell said. “And if not, a new majority will be taking this matter up and sending it to the president.”

The issue has taken center stage in the waning days of this Congress as both parties hope to boost the prospects of their Senate runoff candidates in Louisiana.

Supporters of the bill seemed to have 59 votes to advance it but were still looking for a 60th. Maine independent Angus King announced Tuesday that he would oppose the bill despite what he described as his frustration over Obama’s refusal to make a decision on it.

“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. “I urge the president to make a decision soon, and, if he doesn’t, I look forward to working with Congress to put a timeframe on this decision.”

The pending vote puts pressure on Obama to approve the pipeline, which he has resisted in the past. Environmentalists have pressed him to reject the pipeline as proof of his commitment to curb global warming, even though a State Department environmental review said it would not worsen the problem. The oil industry, labor unions and Republicans have called on Obama to approve it, arguing that it would create jobs and reduce oil imports from the Middle East.

“Today we will have that debate again and I hope at the end of the day we will have 60 votes we need,” said Sen. John Hoeven, R-N.D., the lead sponsor of the bill as he opened debate on the bill Tuesday. “The time has come to act and that is what this legislation is all about.”

The bill has fallen victim to Senate gridlock in the recent past, but Landrieu, with her political career at stake, launched an effort last week to find enough Democratic converts for passage.

“Let the record be clear forever that this debate would not be before this body if not for Sen. Landrieu’s insistence,” said Sen. Barbara Boxer, D-Calif., who led the opposition to the bill Tuesday. Boxer will be replaced as chair of the environment committee by climate-change denier and pipeline supporter Sen. James Inhofe of Oklahoma next year.

The vote offers a preview of what is ahead for Obama on energy and environmental issues when the Republicans take control of both houses of Congress.

For six years, the fate of the Keystone XL oil pipeline has languished amid debates over global warming and the country’s energy security. The latest delay came after a lawsuit was filed in Nebraska over its route.

The White House has issued veto threats of similar bills, and issued three veto threats on House bills targeting the Environmental Protection Agency that were slated for votes on Tuesday. But it did not issue a formal veto threat on the Keystone bill. Both administration officials and Obama have indicated a veto is likely. Landrieu said last week that neither the Senate nor House has the two-thirds majority needed to overcome a veto.

The proposed crude-oil pipeline, which would run 1,179 miles from the Canadian tar sands to Gulf coast refineries, has been the subject of a fierce struggle between environmentalists and energy advocates ever since Calgary-based TransCanada proposed it in 2008.

The Obama administration’s delays have caused friction between the U.S. and Canada, which needs infrastructure in place to export its growing oil sands production.