The Pentagon is threatening to start closing down unneeded military bases unilaterally if Congress continues to refuse to launch a new Base Closure and Realignment Commission.

The defiant move would raise a host of legal questions and set the stage for a first-of-its-kind political showdown between the Defense Department and Capitol Hill, which historically for years have coordinated closely on such the controversial issues of picking which bases to close.

Military leaders are eager to save money by shuttering some underused bases in an effort to save money. But the move is that’s extremely unpopular on Capitol Hill, where many lawmakers fear the process would jeopardize put government jobs in their own districts at risk.

For years Congress has refused the Defense Department’s repeated requests to create a bi-partisan commission, known as a BRAC, that would — with help from military officials — identify the bases most appropriate for closure and present lawmakers with a single, comprehensive plan slate of closures for an up-or-down vote.

Last week, the Pentagon dialed ratcheted up its pressure on Capitol Hill on April 15, sending a report to Congress showing that the military currently maintains about 22 percent more installation space and infrastructure than the current force requires. The report was careful to avoid mention of any specific bases, but indicates the Army and the Air Force have the most glut.

Coincidentally, on Tuesday House lawmakers introduced legislation calling for 27,000 additional troops — mostly Army guardsmen and reservists.

The Pentagon's report threatened that the Defense Department would act on its own if Congress continues to reject the Pentagon's request for a new BRAC.

"The alternative to BRAC is either attempting to close individual installations, or making reductions to personnel and shuttering or mothballing parts of installations across the country," according to the Pentagon's 20-page report to Congress, which. The report echoesd some fine-print in the Defense Department's 2017 this year’s Pentagon budget request unveiled earlier this year.

That document, released in February, says "the need to reduce unneeded facilities is so critical that, in the absence of authorization of a new round of BRAC, the Department will explore any and all authorities that Congress has provided to eliminate wasteful infrastructure," according to the Defense Department's 2017 budget request released in February."

Technically, experts say, the Defense Department’s only legal requirement is to notify Congress about base closures. However, But before it could close closing a specific base, federal law would require a detailed — and time-consuming — study of the closure's environmental and socioeconomic impact, giving Congress plenty of time to cast a vote blocking the move.

"DoD has some authorities. They aren't great, but they are there, whereby DoD can shutter, reduce or eliminate facilities," said Mark Cancian, a military force structure expert with the Center for Strategic and International Studies, or CSIS, a think tank in Washington.

The threat alone could upend the intense politics surrounding of the issue. If the Defense Department put forth a proposal with a list of specific bases targeted for closure, many of the lawmakers unaffected by the specific proposal might breathe a sigh of relief and drop their instinctive opposition.

"One of the dynamics around BRAC is that before a BRAC happens, everyone sees themselves as a potential loser. Once people know what the actual proposal is, there are more winners than losers," said Andrew Hunter, a former Capitol Hill staffer who is now a defense expert with CSIS.

"Once a list is out, there is a strong compelling logic for the people on the list to fight it. But there is also a logic for the people who aren't on the list to support it," Hunter said.

Cancian said the Pentagon's threat may be a strategic effort to convince Congress to approve a BRAC for fear that military officials will take unilateral action.

"I think DoD's purpose is to use this as a way to get a BRAC ... the department trying to move the Congress along," he said, noting that a .A similar threat was floated about 25 years ago by then-Defense Secretary Dick Cheney, Cancian said

Military officials say implementing a new BRAC would eventually save the Defense Department about $2 billion annually.

The size of the active-duty military has decreased by nearly half since the end of the Cold War, and many facilities across the country are underused, unnecessary and require maintenance that is draining the military budget.

Congress has convened base closure commissions five times in recent years, in 1988, 1991, 1993, 1995 and 2005. Typically BRACs aim to reduce total capacity by about 5 percent.