When the Senate returns Monday, the upper chamber could play host to the first substantive energy debate it has had since 2009 and lay the groundwork for a vote on the Keystone XL oil sands pipeline.

Lawmakers were busy over the weekend hammering out a possible deal on amendments to an energy-efficiency bill from Sens. Jeanne Shaheen, D-N.H., and Rob Portman, R-Ohio.

Simultaneously, backers of the Keystone XL pipeline were optimistic that Senate Majority Leader Harry Reid, D-Nev., would soon call for a vote to approve it. Reid told reporters that he's "75, 80 percent" certain lawmakers could broker a deal for a vote.

Reid has apparently made the calculation that allowing a vote would offer red meat to some centrist Democrats running tight re-election races, giving them a chance to say they support the pipeline. He told the Wall Street Journal a vote "would not be bad" for Democrats.

But the legislation allowing congressional approval of the $5.4 billion Canada-to-Texas project needs four more Democratic supporters to clear procedural hurdles to hold a vote.

Sen. Mary Landrieu, D-La., who is co-sponsoring the Keystone XL bill, which has 56 co-sponsors, 11 of whom are Democrats, insisted the vote would go forward.

"It's going to be a standalone vote," Landrieu said.

But if Landrieu and other Keystone XL backers cannot corral 60 senators to proceed to a vote, Democrats run the risk of taking the blame for punting on the pipeline.

As lawmakers left Washington last week, four Democrats were considered in play: Michael Bennet and Mark Udall of Colorado, Tom Carper of Delaware, and Bob Casey of Pennsylvania.

Bennet, Carper and Casey voted in favor of a non-binding measure that supported Keystone XL in March 2013.

Bennet recently told the Wall Street Journal that he supports Keystone XL. Casey has voted for it twice, and could do so again.

Udall hasn't taken a position on the pipeline, not least because he's facing a tough re-election contest in purple Colorado. His challenger, Republican Rep. Cory Gardner, supports Keystone XL.

Pressure is sure to mount on all four of those senators. Jamie Henn, a spokesman for climate advocacy group 350.org, said of Carper that his organization plans to "pay him an office visit." And Udall will likely catch heat from both the pipeline's boosters and detractors leading up to his decision.

Still, some Republicans prefer to see Keystone XL as an amendment to the energy-efficiency bill.

Senate Minority Whip John Cornyn, R-Texas, said President Obama would veto a standalone Keystone XL vote. Given the current struggle to get the 60 supporters, securing 67 to override a veto looks unlikely.

"That's not good enough," Cornyn said of a separate Keystone XL vote.

Other Republicans are less insistent on Keystone XL as an amendment to the efficiency bill, but they're loathe to support that measure unless Reid allows other GOP amendments.

Some of those amendments include scuttling proposed carbon emissions regulations for new power plants and expediting liquefied natural gas exports. If Reid allows those measures, it's likely to come with the caveat that they reach 60 votes for passage.

That's been the hold-up for the past three years, Republicans say, but they're hoping times have changed.

"There's about five amendments we're talking about," Sen. John Hoeven, R-N.D., told reporters last week in the Capitol. "We'll proceed to the energy-efficiency bill, I think, but to really get into it and advance the bill we're going to have an agreement on not just Keystone, but these other things."