For more than a year, a band of lawmakers has been fighting to save one of the Pentagon’s most ferocious fighters: the A-10 Warthog.

Now, they have a little more ammunition.

A report issued last week by the Government Accountability Office says that the Air Force “has not fully assessed the cost savings” it could gain by retiring the aircraft and that divestment would leave the military unable to fully provide what’s called close air support, the military tactic that is the A-10’s primary mission.

Supporters of the A-10, such as Sen. John McCain (R-Ariz.), chairman of the Senate Armed Services Committee, pounced on the report, saying in a statement that “any premature divestment of the A-10 would not only fail to achieve the Air Force’s purported cost savings, but also leave us with a serious capability gap that could put the lives of American soldiers in danger.”

The A-10 is the in-the-trenches-grunt of the Air Force’s fleet, an ugly, snub-nosed plane, nicknamed for its tenacity. Built initially to take out Russian tanks during the Cold War, it has become a darling of the military, beloved for its ability to fly low and slow over the battlefield and rescue soldiers who are in close contact with the enemy.

Armed with a 30 mm, rip-snorting cannon, and a “titanium bathtub” belly designed to absorb ground fire, it was also used heavily in Iraq and Afghanistan. And, as McCain pointed out, it is “playing an important role in the fight against ISIS in Iraq and Syria and in NATO’s effort to deter Russian aggression in Eastern Europe.”

But it is old and its close-air-support duty is its sole purpose, Air Force officials have said. The F-35 Joint Strike Fighter, which is still in development, will take over that role, they said. With a tight budget, the Air Force has said it has no choice but to retire the A-10, saving $4.2 billion over five years.

But the GAO called that figure into question, saying that “without a reliable cost estimate, the Air Force does not have a complete picture of the savings it would generate by divesting the A-10.” And the report said that as the Air Force introduces the F-35 into the fleet, the close air support “capability will be limited for several years.”

In response to GAO’s report, Air Force Secretary Deborah Lee James wrote that the watchdog failed to take into account the “potential risks to air superiority and global strike that could be created by the added cost of retaining the A-10 fleet.”

Keeping the Warthogs could also further delay the F-35 program, James said, because the Air Force would not be able to transfer hundreds of maintenance workers from the A-10 to the F-35, as originally planned.

Lawmakers wrote a provision into the Pentagon’s budget plan this year that would prevent the Air Force from retiring any A-10s, meaning the fight over the aircraft will likely continue for at least another year.