WASHINGTON — The US House and Senate armed services committees are working to identify $5 billion to cut from the defense budget as part of a larger budget pact between Congress and the White House, leaving procurement programs vulnerable to reductions.

The Pentagon will get nearly everything it wanted in the budget deal — minus but $5 billion — leaving defense watchers to speculate how the revised and rewritten policy bill will play out.  

Reacting to the runaway aerostat in the news Wednesday, Kingston Reif, of the Washington-based Arms Control Association, tweeted out: "To appropriators looking to find part of that $5 billion in savings to meet revised [budget caps]: FY16 request for JLENS exercise is $40.6 million."

Key authorizers have yet to publicly name the source of the cuts. Echoing SASC Chair Sen. John McCain, R-Ariz., who on Tuesday could not say where the cuts would come from, HASC Chairman Rep. Mac Thornberry, R-Texas, said lawmakers are still working on the answers.

"It will come out of muscle, there isn't just fat you can chip away and say this doesn't matter, it will matter, it will be significant,"  Thornberry told reporters Wednesday. "It will be a $5 billion reduction in the spending that is authorized and that was the agreement, which I support."

The armed services committees — which pen the National Defense Authorization Act, a policy bill — are working with their counterparts in the appropriations committees to find the cuts, Thornberry said.

This dovetails with preparations to advance the recently vetoed policy legislation, either by overriding the president’s veto of the bill, or advancing a new version that simply adjusts the dollars. While the method remains to be seen, Thornberry was confident the various policy provisions it would be preserved.

"Either path would reflect the $5 billion less, and that would be the only change," Thornberry said. "If we have a veto override, we have to have separate legislation to make those adjustments, so it's a two-step process. I don't know for sure which path we'll go."

House leaders had scheduled a veto override vote for next week, but the status of that maneuver is now unclear.

Last week, Obama sent the $612 billion bill back to Congress, saying it "fails to authorize funding for our national defense in a fiscally responsible manner."

At issue was about $38 billion that Republican lawmakers added to the Pentagon's overseas contingency operations fund in order to get around mandatory spending caps enacted by Congress for fiscal 2016. Republicans had argued that the issue was better settled in the separate appropriations process, not the authorization process.

But the two-year budget deal unveiled Monday night essentially solves that problem, McCain said. Under the plan, fiscal 2016 defense spending would be raised to $607 billion, necessitating some changes in the totals of the authorization bill language.

One Senate staffer said lawmakers are likely to target the programmatic increases derived from the Pentagon's "unfunded requirements list," meaning the Pentagon wish list items added to the president's budget request, or other big ticket programs.

Obvious targets, the aide said, include the DDG-51, which was plussed up $400 million, part of $1 billion above the budget request; the Army's $128 million request for eight additional UH-60M Black Hawk helicopters for the Guard; $314 million for Stryker vehicle lethality upgrades; and the Distributed Common Ground System - Army,  an intelligence system faulted in the AC-130 strike on an Afghan hospital.

"Bottom line, procurement accounts will take the brunt of the losses," the staff member said.

Michael Herson, president of American Defense International, a Washington defense lobbying firm, said lawmakers from the House and Senate, authorizers and appropriators, will be moving quickly to reconcile the various areas where they propose to cut. Authorizers in particular want to get their bill on the floor soon.

Procurement accounts are vulnerable, Herson said, but so are operations and maintenance accounts, and research and development programs.

"It's unlikely you'll see an across-the-board cut where they share the pain everywhere, because frankly it looks too easy to cut like that, across many programs and services and will wreak too much havoc," Herson said. "There's going to be some pain, but where that pain will be is to be determined."

On the flip side, Mackenzie Eaglen, an American Enterprise Institute analyst and former congressional defense aide, agreed said procurement is one place where lawmakers will identify cuts for a revised and rewritten bill, and that the cuts are likely to be spread around based on input from Pentagon leaders and service chiefs. 

"They'll probably go through the plus-ups that were in the unfunded requirements list first, and then seek to spread the pain and simply lessen some of those, as well as other increases elsewhere, but not let anything take too big a hit," Eaglen said

Todd Harrison, a senior fellow at the Center for Strategic and International Studies, said lawmakers will likely look to acquisitions programs for reductions, potentially likely slowing them down. Typically the low-hanging fruit when looking for cuts is prior-year money that hasn't already been spent, but "that can probably only yield a few billion dollars." 

Staff Writer Leo Shane contributed to this report.

Twitter: @reporterjoe