A Senate vote on the Keystone XL oil sands pipeline is close to falling apart.

Senate Majority Leader Harry Reid indicated that vote was tied to whether Republicans would allow an up-or-down vote on energy-efficiency legislation sponsored by Sens. Jeanne Shaheen, D-N.H., and Rob Portman, R-Ohio, to go forward.

"Well, at this stage they're tied together," the Nevada Democrat said Tuesday during a press conference.

Reid said he wouldn't allow Republicans to attach four or five particular amendments to the efficiency bill. GOP lawmakers are likely to peel off support, jeopardizing the chance of a Keystone XL vote since Reid won't allow a Keystone XL vote without a vote on the energy-efficiency bill.

Reid insisted Republicans kept shifting their demands. The Nevada Democrat told reporters in the Capitol that GOP leadership had wanted only a standalone vote on the Keystone XL oil sands pipeline, which he has agreed to.

"If they want to vote Keystone, they can have a vote on Keystone. But enough is enough. This is a good bill," Reid said of the efficiency bill.

The bill to approve the Canada-to-Texas pipeline has 56 co-sponsors and support of one other Democrat, leaving boosters three votes shy of the 60 needed to hold a vote.

Republicans see the separate Keystone XL vote as a way to force President Obama's hand on the pipeline, which has been under federal review for more than five years, and to tie Democrats to the administration in the likely case of a veto. Reid backing off of a Keystone XL might also give them ammunition.

“Here you have a project that the American people support overwhelmingly, that would create thousands of jobs when we’ve rarely needed them more, and that would pass Congress easily if the majority leader would just allow a vote," said Senate Minority Leader Mitch McConnell, R-Ky.

“But he won’t. Because the Far Left won’t let him. And if we do get a vote, the Democrat leadership would be sure to filibuster against the jobs the Keystone XL Pipeline would create."

But 11 of the Keystone XL bill's backers are Democrats, some of whom are facing tight re-election contests in red-leaning states. They view the vote as a potential boost to their electoral prospects.

GOP lawmakers are holding firm against the efficiency bill, as they don't want to be viewed as caving to Reid. One Senate GOP aide was "confident" the party would unify to block the measure. Democrats would need to find five Republicans to cross the aisle to pass the legislation.

That's looking difficult. Sen. John Hoeven, R-N.D., one of the bill's co-sponsors, said before Reid's remarks that his support was contingent on some or all of those amendments making their way to the floor. On top of that, Portman spokeswoman Caitlin Dunn declined to comment on whether the Ohio Republican would vote for his own bill without amendments.

Among other Republican co-sponsors, Robert Dillon, the energy committee spokesman for Sen. Lisa Murkowski, said he didn't know how the Alaska Republican would vote. Kevin Kelley, a spokesman for Sen. Susan Collins, said the Maine Republican was undecided. The offices of the bill's other GOP co-sponsors -- Sens. Kelly Ayotte, of New Hampshire, Johnny Isakson, of Georgia, and Roger Wicker, of Mississippi -- did not immediately return requests for comment.

Republicans put the blame for the impasse on Reid, as they said they wanted several specific amendments all along.

Republicans said the last chance to debate energy policy on the floor was in 2007, the last time substantive energy legislation hit the floor. They're pushing amendments that would block a carbon tax, speed up approvals of liquefied natural gas exports and scuttle proposed carbon emissions rules for power plants.

The efficiency bill, which cleared a procedural hurdle Tuesday and could come up for a vote this week, is designed to spur investment in energy-saving technology at industrial facilities, the federal government and buildings. But it's been a magnet for controversial amendments, which Republicans accused Reid of blocking.

"We're now reduced to asking for five energy amendments on an energy bill," Sen. Roy Blunt, R-Mo., said Tuesday during a GOP leadership press conference in the Capitol. "The Harry Reid gag order is not good for the Senate and it's not good for the country."

Reid said that Democrats had already greased the bill since a push by Sen. David Vitter, R-La., to attach an amendment gutting federal health care contributions to lawmakers, Capitol Hill staff and appointees derailed the legislation last fall. Democrats also have pushed back against the Republican-pushed energy amendments because they say they're not germane to the efficiency bill.

That's not to say Democrats haven't budged on some things.

Shaheen and Portman, in a move to gain more GOP support, reintroduced the legislation after adding several bipartisan amendments, which environmental groups said watered the bill down. Without allowing the other Republican amendments to go forward, though, the bill's prospects are dim.

"I'm reminded of one of Mick Jagger's songs where he has a line that says, 'You can't always get what you want, but you try sometimes to get what you need.' Legislation is the art of compromise, which seems to be far out of style around here. The question is, can you get what you need?" Reid said.

This story was published at 5:26 a.m. and has been updated to clarify the prospects of a possible Keystone XL vote.