The controversial legislation, which the New York Democrat said she would continue to fight for, would strip commanders of the power to decide which sexual assault cases are prosecuted.
The Senate voted down a crucial motion to proceed on Sen. Kirsten Gillibrand's bill to combat sexual assault in the military, closing the door on the long-fought legislation for now.
The vote wraps up several months of fits and starts on the controversial legislation. Gillibrand's bill would strip commanders of the power to decide which sexual assault cases are prosecuted and is vehemently opposed by the Pentagon's top brass.
The New York Democrat, who argued passionately on the floor Thursday, has vowed to keep fighting for her legislation, which is championed by victim advocacy organizations.
The Senate was expected to easily pass a competing, non-controversial measure from Sen. Claire McCaskill, D-Mo., that would build on other reforms recently adopted in last year's defense authorization act.
Gilibrand's bill attracted 55 supporters but failed to overcome a procedural hurdle requiring 60 votes, with the Senate voting 55 to 45. Had the Senate approved the cloture vote, it would have cleared a path for a vote on final passage, which would have required only a simple majority.