Green-energy advocates are scrambling behind the scenes to prevent what they say could be a devastating blow to the military’s biofuels initiatives. And the clock is ticking.

The Senate is expected to consider the defense authorization bill on the floor this summer — and the ensuing debate could be the last chance to block amendments that would severely restrict the Defense Department’s ability to purchase biofuels and other forms of alternative energy.

The House has already approved its version of the bill, including similar restrictive provisions for military biofuels.

Sen. Carl Levin (D-Mich.), chairman of the Armed Services Committee and a supporter of the biofuels programs, told POLITICO he expects a tough floor fight over the issue, which has become increasingly political since President Barack Obama cited it in his State of the Union address. “I expect there’d be an effort to restore that role for the military since it’s a security issue,” Levin said.

Many Republicans view the programs as a waste of money. In December, for instance, the Navy purchased 450,000 gallons of biofuels at $26.75 a gallon, POLITICO Pro reported. In comparison, the cost of petroleum is just a few dollars per gallon.

For those who support the initiatives, though, the high cost of biofuels is a necessary investment in a promising industry that could one day produce clean energy at a cost comparable or possibly cheaper than fossil fuels.

“Those who question why the Navy should be leading on energy should study their history,” Navy Secretary Ray Mabus said in a statement to the Senate Armed Services Committee. “The Navy has always led in new forms of energy, shifting from wind to coal-powered steam in the middle of the 19th century, from coal to oil in the early 20th century and pioneering nuclear power in the middle of the 20th century.”

The Democratic-controlled committee delivered a surprising blow to the Navy’s energy plans last month when it approved amendments — sponsored by Arizona Sen. John McCain, the committee’s ranking Republican, and Sen. Jim Inhofe (R-Okla.) — that would all but prohibit the Pentagon from building biofuel refineries or purchasing alternative fuels.

The amendments passed 13-12 during a closed-door markup — in part, because Sen. Susan Collins (R-Maine), who plans to oppose the amendments on the Senate floor, was absent during the votes because she had to monitor a fire aboard the USS Miami, a nuclear-powered submarine that was docked at a Maine shipyard.

“Much to my surprise, the vote was called while I was gone,” Collins told POLITICO. “My position is clear that I’m in favor of biofuels.”

In addition, two Democrats — Sens. Jim Webb of Virginia and Joe Manchin of West Virginia, both from coal states — broke with their party to support the amendments.