The fate of the usually bipartisan defense authorization bill could hinge on the political fight over spending on nonmilitary programs next year.
On Thursday, the Senate approved its draft of the annual authorization measure, a policy budget bill that includes a host of service member compensation measures and a massive overhaul of the military retirement system.
The legislation passed with 71 votes, more than enough to override a threatened presidential veto of the measure. But Republican supporters of the bill say they don't think that makes its future any more secure.
"I'm very pleased with 71 votes, but you don't know what pressures may be exerted in the case of a veto," said Sen. John McCain, R-Ariz., chairman of the Senate Armed Services Committee.
"I never understood why (the president) threatened a veto to start with. To not have this defense auth can really damage our ability to defend the nation."
White House officials have promised a veto on the measure — and a host of other appropriations bills — over moves by Hill Republicans to use temporary war funds to get around mandatory spending caps on defense funding next year.
President Obama and congressional Democrats want a compromise that lifts the spending limits for defense and nondefense spending. In a letter to the Senate earlier this month, White House officials said the war funding move "ignores the long-term connection between national security and economic security" and warrants a veto.
A day before the Senate vote, Defense Secretary Ash Carter supported the veto threat even though the Republicans' plan would fund military programs at the level administration officials requested earlier this year.
Republicans have accused their political rivals of holding troops' pay hostage to give more money to the Internal Revenue Service.
Following the Thursday vote, McCain said he hopes the authorization bill can make it through conference committee in a few weeks, and be brought up for a final vote in the House and Senate before Congress heads home for its August recess.
House Armed Services Committee Chairman Rep. Mac Thornberry, R-Texas, expressed similar optimism earlier in the week, noting no major points of contention between the chambers' different authorization drafts.
But that speedy work would be moot if the White House vetoes the legislation. Thornberry would not speculate what lawmakers would have to do to keep alive their 53-year streak of adopting the annual budget bill if Obama kills the legislation, instead hoping that he will drop his hard stance on the funding mechanism fight.
The House version of the authorization bill fell 21 votes short of what would be needed to override a presidential veto, with strong Democratic opposition to the overseas contingency fund plus-up.
McCain said he met with Carter hours before his chamber's vote to discuss ways to move the legislation forward in the face of White House opposition, and said he is still optimistic that a reasonable middle ground can be found.
But he also expressed exasperation at the possibility of Obama blocking the bill over unrelated agency funding.
White House spokesman John Earnest mirrored that frustration a day earlier, when asked if Obama was serious about the politically risky move of vetoing a defense budget bill.
"The president does not believe that (plan) is consistent with any sort of serious approach to national security," he told reporters. "That is the principal objection of the way that Republicans are currently trying to move this legislation through Congress."
Obama has threatened to veto every defense authorization bill of his presidency so far, but has not followed through on any of those vows.
He has vetoed only four bills thus far in his six years in office. The only president to veto fewer than 10 bills in the last 100 years was Warren Harding, who scuttled five during his 2.5-year term.