Sen. Kirsten Gillibrand, here at a news conference on sexual assault with Sen. Rand Paul and Sen. Chuck Grassley, had pushed hard for passage of the bill for a year. Mark Wilson/Getty Images

The Senate blocked a bill Thursday that would have stripped senior U.S. military commanders of their authority to prosecute rapes and other serious offenses in the ranks — and instead put prosecutorial power into the hands of military trial lawyers who would operate outside of the military chain of command.

The vote on Thursday was 55–45, short of the 60 necessary to move ahead on the legislation.

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, sponsored by Sen. Kirsten Gillibrand, insist that far-reaching changes in the Uniform Code of Military Justice are necessary to curb the scourge of rapes and sexual assaults within the armed forces.

One of those changes, they argue, would be to ensure that there is less potential for personal bias with military commanders overseeing charges of sexual assault brought against their subordinates.

Susan Burke, an attorney who supported Gillibrand's bill, told Al Jazeera that such a change is simply logical.

"The best justice is blind justice," she said. "If you’re sitting on a jury, you’re excused if you know [the defendant]."

Gillbrand expressed her disappointment after the vote in a press release, saying, "I always hoped we could do the right thing here — and deliver a military justice system that is free from bias and conflict of interest — a military justice system that is worthy of the brave men and women who fight for us.

"But today the Senate turned its back on a majority of its members," she said.

The showdown vote capped a nearly yearlong campaign by Gillibrand but was unlikely to be the final word.

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. 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.

In recent years, the military has struggled with numerous reports of sexual assault.

On Thursday, for instance, Army Brig. Gen. Jeffrey Sinclair, who was accused of sexual assault, pleaded guilty to three lesser charges, including possessing a hoard of pornography, a violation of Army regulations. Sinclair is believed to be the most senior member of the U.S. military to face trial on sexual assault charges.

Late last year, after much debate, Congress passed numerous changes to the military's legal system. But the reforms didn't go far enough for many lawmakers.

Under Gillibrand's proposal, the decision to take serious crimes to courts-martial would be removed from commanders and given to seasoned military trial lawyers who have prosecutorial experience and would operate out of a newly established office independent of the chain of command.

The legislation, Gillibrand said in a recent interview, would spark the cultural shift needed to create a climate in which victims have the confidence to step forward and report sex crimes without the fear of retaliation.

With commanders making the call, there is the chance a personal bias may influence the decision, proponents of Gillibrand's bill have argued.

The dispute hinges on the role that senior military commanders play within the Uniform Code of Military Justice. The Defense Department system is completely separate from civilian courts, and with the code, commanders are vested with substantial authority to decide when and how to deal with crimes committed by service members.

That power to punish or pardon has been a principal tenet of military law dating back more than two centuries. It's rooted in the military's conviction that commanders must have the ability to discipline the troops they lead in peacetime and war. Undercutting that role, top Defense Department officials have warned, would send a message that there is a lack of faith in the officer corps, and that in turn would undermine the efficiency and effectiveness of the armed forces.

Al Jazeera and wire services