The proposal by Sen. Kirsten Gillibrand, D-N.Y., appeared headed for approval before the Thanksgiving recess, The Hill reported Sunday.
However, a desire by leaders of the House and Senate to limit amendments to assure passage of the authorization package by Friday probably won't allow for major changes, Senate aides say.
Passage of the defense bill is considered a "must" because it contains provisions such as pay hikes for troops and the payment of death benefits.
Rep. Adam Smith, D-Wash., a member of the House Armed Services Committee, said the House and Senate were going to "ping-pong" the defense bill in a maneuver that would bypass the Senate's normal floor vote procedures and a formal conference committee.
Gillibrand's proposal would remove the military chain of command from involvement in the investigation or prosecution of sexual assault cases reported by service members. She had obtained 53 of the required 60 votes for passage in the Senate when Republicans blocked a vote over objections Senate Majority Leader Harry Reid, D-Nev., was not allowing their amendments to be considered.
Her amendment would have to pass before the House adjourns.
Gillibrand spokeswoman Bethany Lesser said the senator was "confident" she would eventually get a vote on her proposal, even if it wasn't part of the defense bill.