Updated

The Senate on Thursday blocked a bill that would have stripped senior military commanders of their authority to prosecute rapes and other serious offenses, capping an emotional, nearly yearlong fight over how best to curb sexual assault in the ranks.

The vote was 55-45, short of the 60 necessary to move ahead on the legislation sponsored by Sen. Kirsten Gillibrand, D-N.Y. Defeated but unbowed, the senator received hugs from Sens. Amy Klobuchar, D-Minn., and Jeanne Shaheen, D-N.H., after the vote.

The Pentagon's leadership vigorously opposed the measure, arguing that officers should have more responsibility, not less, for the conduct of the men and women they lead.

Proponents of the bill insisted that far-reaching changes in the Uniform Code of Military Justice are necessary to curb a scourge of rapes and sexual assaults.

Gillibrand's effort bitterly divided the Senate in a battle that smashed conventional lines on gender and political party.

Conservative Sens. Ted Cruz of Texas and Rand Paul of Kentucky backed her effort, while the chairman of the Armed Services Committee, Democratic Sen. Carl Levin of Michigan, opposed the measure.

Although the vote sent the bill back to the Senate calendar, it was unlikely to be the final word. Gillibrand was expected to pursue the issue this spring when the Armed Services Committee begins work on a sweeping defense policy bill for the 2015 fiscal year.

"The people who don't trust the chain of the command are the victims," Gillibrand told her colleagues during the Senate debate.

One by one, proponents and opponents of her bill stood on the Senate floor and passionately argued based on personal experiences, growing frustration with what they dismissed as fixes around the edges and horrific stories from the ranks.

"The current system is failing the men and women in uniform," said one of the Senate's newest members, John Walsh, D-Mont., who spent 33 years in the Montana National Guard and is the first Iraq War veteran in the body. "We have moved too slowly."

Countering that argument was Sen. Jack Reed, D-R.I., a West Point graduate who served in the 82nd Airborne Division as an infantry platoon leader and company commander.

Reed said stripping commanders of the authority to discipline the troops would be "detrimental to the effectiveness of the force and common goal to reduce sexual assault."

The issue bitterly divided the Senate's 20 female members.

"How many more victims are required to suffer before we act?" asked Sen. Susan Collins, R-Maine, a backer of Gillibrand's effort. "How many more lives must be ruined before we act?"

Sen. Claire McCaskill, D-Mo., a vehement opponent of the bill and a former prosecutor, said more cases are going to trial over the objections of prosecutors as commanders are deciding to press ahead with charges.

"We can't let the commanders walk away," McCaskill said.

The military has struggled increasingly in recent years with the sexual assault issue. The Pentagon estimated that 26,000 members of the military may have been sexually assaulted in 2012, the most recent numbers available, though thousands were afraid to come forward for fear of inaction or retribution.

After blocking Gillibrand's bill, the Senate moved toward overwhelmingly passage of a measure sponsored by McCaskill and two Republican senators -- Kelly Ayotte of New Hampshire and Deb Fischer of Nebraska. That bill would eliminate the "good solider defense" in cases unless it is directly connected to the crime. And it would allow sexual assault victims to challenge their discharges or separation from service.

The bill also calls for a civilian service secretary review if a prosecutor and commander disagree over whether to litigate a case.

Three months ago, Congress cleared and President Barack Obama signed a defense policy bill that included several changes to military law, including stripping commanders of their ability to overturn jury convictions and criminalizing retaliation against victims who come forward about sexual assaults.

Last June, members of the Senate Armed Services Committee, especially Gillibrand and McCaskill, grilled the Joint Chiefs of Staff about whether the military's mostly male leadership understands differences between relatively minor sexual offenses and serious crimes that deserve swift and decisive justice.

"Not every single commander necessarily wants women in the force. Not every single commander believes what a sexual assault is. Not every single commander can distinguish between a slap on the ass and a rape because they merge all of these crimes together," Gillibrand said.

The Pentagon came under pressure last month to disclose more information about how sexual assault cases are adjudicated following an Associated Press investigation that found a pattern of inconsistent judgments and light penalties for sexual assaults at U.S. bases in Japan.

Gillibrand, who chairs the Senate Armed Services personnel subcommittee, called on Defense Secretary Chuck Hagel in a Feb. 10 letter to turn over case information from four major U.S. bases: Fort Hood in Texas, Naval Station Norfolk in Virginia, Camp Pendleton Marine Corps base in California and Wright-Patterson Air Force Base in Ohio. Such records would shed more light on how military commanders make decisions about courts-martial and punishments in sexual assault cases and whether the inconsistent judgments seen in Japan are more widespread.

The AP's investigation, which was based on hundreds of internal military documents, found that what appeared to be strong cases were often reduced to lesser charges. Suspects were unlikely to serve time even when military authorities agreed a crime had been committed. In two rape cases, commanders overruled recommendations to court-martial the accused and dropped the charges instead.