After three weeks of heavy lobbying in which the Navy, veterans and advocacy groups have linked the service's biofuels program with national security, by the end of last week Sen. John McCain (R-Ariz.) had enough.

In a fiery letter sent Friday afternoon to Navy Secretary Ray Mabus, the decorated combat veteran and ranking member of the Senate Armed Services Committee defended his efforts to rein in the Navy's program and accused Mabus of "politically-driven" maneuvers that "result in a real cost to the readiness and safety of our Sailors and Marines."

McCain authored one of two hotly contested provisions in the 2013 defense policy bill that aims to restrict the Navy's efforts to build a cost-competitive domestic biofuels market. The Navy contends that such a market would offer it a reprieve from volatile oil prices, which can force the military to take money out of weapons programs or maintenance funds in order to cover higher-than-expected fuel costs.

McCain and some other congressional Republicans have balked at the price the Pentagon has paid for the fuel, especially as the military faces $487 billion in cuts over the next decade. The Navy paid $12 million for the 450,000 gallons of biofuel it used earlier this month to demonstrate its "Great Green Fleet" in exercises off the coast of Hawaii (Greenwire, July 19). Mixed half-and-half with traditional petroleum-based fuel, the biofuel blend cost roughly $15 a gallon, according to Mabus.

"You are the Secretary of the Navy, not the Secretary of Energy," McCain wrote Friday. "I strongly encourage you to marshal the time and resources of your team to avert serious threats to the core mission and capabilities of the Department of the Navy, instead of spending defense dollars to advocate your view of our national energy priorities."

He also accused Mabus of confounding the benefits that energy efficiency brings with the benefits of biofuels and of having "ordered" senior Navy leaders to advocate for the program. The Navy's top officer, Adm. Jonathan Greenert, and his top energy officer spoke on Capitol Hill earlier this month about the service's concerns about the provisions at the request of senators opposing the language.

The Navy has said it will not purchase operational quantities of alternative fuels until they are cost-competitive with petroleum. But McCain and others argue that the "Green Fleet" purchase was in fact just that.

In a statement Friday, Sen. James Inhofe (R-Okla.) pointed out that the $12 million fuel buy came out of "operations and maintenance funds" rather than the Pentagon's research and development budget line.

"Tapping into [operations and maintenance] funds for last week's demonstration means less funding for training, supplies, equipment, repairs, and over all readiness putting at risk the lives of our sailors," Inhofe said.

But Navy officials say that the event -- which was the first time an air-to-air replenishment was done with biofuels and the first time that they ran through the entire fuel distribution system -- was more than just show.

"We are in the process of collecting feedback from every sailor whose equipment touched the fuels," Rick Kamin, the Navy's fuel lead, said at the event.

Majority Leader Harry Reid's (D-Nev.) decision to bring cybersecurity legislation to the floor this week means that the defense policy bill -- and the biofuels provisions it contains -- will likely be sidelined until lawmakers return from summer recess in September.