The fate of a Senate energy-efficiency bill might come down to the Keystone XL oil sands pipeline.

Democrats on Thursday said they planned to offer a standalone vote on whether to approve the $5.4 billion Canada-to-Texas project, fearing that attaching it to an energy-efficiency bill they plan to bring to the floor next week would sink the measure.

"I think that there's a 75, 80 percent chance that we can work something out on Keystone," Senate Majority Leader Harry Reid, D-Nev., told reporters Thursday following a meeting with his Democratic colleagues.

But after some insisted on a binding, standalone measure earlier in the week, GOP lawmakers are now demanding a Keystone XL vote, with other priority Republican energy items, as an amendment to the efficiency bill.

Such a move would jeopardize the prospects of the efficiency bill, which is sponsored by Sens. Jeanne Shaheen, D-N.H., and Rob Portman, R-Ohio.

Republicans told reporters a standalone Keystone XL vote wouldn't work because President Obama would probably veto the bill.

"That's not good enough," Senate Minority Whip John Cornyn, R-Texas, told reporters of the standalone Keystone XL vote. "It will never see the light of day. The president's not going to sign it. But if it's part of the underlying energy-efficiency bill, there's a little different calculation."

Even so, Keystone XL backers were having trouble getting enough supporters to even have a vote.

Legislation reintroduced Thursday by Sen. Mary Landrieu, D-La., and John Hoeven, R-N.D., to approve the pipeline netted 56 co-sponsors, 11 of them Democrats. As lawmakers left Washington, Republicans were struggling to bring four other Democrats into the fold.

A standalone Keystone XL vote would give some centrist Democrats facing tough re-election fights, like Landrieu, a chance to side with the pipeline, which, due to several delays, has been under federal review for a cross-border permit for more than five years.

That move could face another obstacle -- Sen. David Vitter, R-La., has pledged to push an amendment to nix federal health care contributions for Congress and appointees. He tried to force a vote on the measure last fall, which derailed the efficiency bill.

Democrats say they don't want a repeat.

"I hope we don't have to deal with this over and over again," Senate Majority Whip Dick Durbin, D-Ill., told reporters in the Capitol.

But it's not just Keystone XL or the Vitter amendment holding up the efficiency bill.

Much like in the previous three years, Republicans want to secure votes on other items, such as expediting liquefied natural gas exports and scuttling proposed carbon emissions regulations on new power plants.

Some of the efficiency bill's seven GOP co-sponsors are threatening to peel off the bill if they can't secure amendments on those measures. That would likely leave Democrats shy of the 60 votes needed to proceed to a vote on the underlying legislation.

"I would probably not vote for cloture upon that circumstance," Hoeven said of the bill coming to the floor lacking those amendments. "I think if we don't get an agreement it's going to be very tough to advance the bill."