WASHINGTON — On the eve of a showdown, Maine’s independent Sen. Angus King left open the possibility Monday that he would support legislation to approve the controversial Keystone XL Pipeline, which appears just shy of the 60 votes it needs to advance.

“I’m a probable no,” King told reporters. Yet several times he refused to rule out voting for the legislation, which has the public support of all 45 of the Senate’s Republicans and 14 of its Democrats.

King made his comments as Louisiana Sen. Mary Landrieu, the bill’s chief Democratic supporter, claimed anew that the measure has the 60 votes. The Republican leading the campaign for the bill, Sen. John Hoeven of North Dakota, was more cautious. “I think we’ll get there,” he said, although he added that he so far has counted only 59 firm supporters.

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. She trails Rep. Bill Cassidy, R-La., in the polls in advance of a Dec. 6 runoff election in Louisiana, but is campaigning as a skilled and seasoned lawmaker with the ability to pass bipartisan legislation that’s vital to her oil-rich state.

The House passed legislation last week to allow construction to proceed, with Cassidy claiming much of the credit.

Even if the bill clears Congress, the White House has discouraged any speculation that President Obama would sign it.

The president has said he wants to allow an environmental review process to be completed and let the Nebraska Supreme Court rule on the legality of a law that allowed state officials to select the pipeline’s proposed route.

Supporters of the measure say the pipeline would provide jobs and ensure that the oil would be processed in the United States. Opponents argue that the project could harm the environment, and say much of the oil would wind up being exported overseas.