U.S. senators voted this morning to allow debate on the so-called Ryan-Murray budget compromise to end, a key procedural decision that permits the upper chamber to pass the deal and — assuming no differences with the House’s version — to go to the White House to become law.

The procedural vote on Dec. 17, to invoke what is called cloture, needed 60 votes in the 100-seat Senate to pass, and it received 67. Such votes require the acquiescence of enough minority Republicans, as the Democratic-Independent caucus only controls 55 seats. In the end, a dozen Republicans voted with Democrats and independents to allow a final vote to occur this week.

That final vote needs only a simple majority of senators to pass the bill, and it is expected before the Senate’s targeted adjournment on Dec. 20. But before then, some members may try to amend the compromise on the Senate floor, which could imperil any deal.

Indeed, as senators were voting on cloture Sen. Jim Inhofe (R-Okla.), the ranking member of the Senate Armed Services Committee, issued a statement that he has joined fellow Republicans Sens. Roger Wicker (Miss.), Lindsey Graham (S.C.), Jeff Sessions (Ala.) and Thad Cochran (Miss.) to introduce an amendment to restore retirement pay increases for current and future military retirees that would be dialed back in the deal as one revenue generator.

The agreement announced last week by House and Senate Budget committee chairmen Rep. Paul Ryan (R-Wis.) and Sen. Patty Murray (D-Wash.) would find $6 billion for its spending and deficit-reduction goals by cutting the cost-of-living increase for military pensioners of working age, i.e., under 62, by 1%. When pensioners reach the traditional retirement age, they would then receive the full, previous amount of annual increases.

But the Republican senators — many of whose southern and central states count legions of current and ex-military personnel — said the move breaks a promise to veterans. “They deserve more from us in their retirement than this agreement provides,” said Graham, who faces a tougher re-election next year than before due to tea party challenges. “Simply put, it doesn’t do enough to protect those who have spent their lives protecting our nation.”

Echoed Cochran, who faces a similar election battle: “There is no justification in first turning to those who have served or are serving in the military and asking them to make new sacrifices.”