Washington (CNN) -- The U.S. Senate on Thursday defeated an impassioned legislative push to reduce the growing problem of sex assault in the armed forces by overhauling the way the military prosecutes serious crimes.
By a vote of 55 to 45, a bill championed by Sen. Kirsten Gillibrand failed to get the 60 votes needed for passage.
The measure would have removed military commanders from deciding whether most serious allegations of wrongdoing by their subordinates should be prosecuted. The responsibility would have been shifted to prosecutors outside the chain of command.
Gillibrand, a New York Democrat, said the change was needed because bias can creep into the decisions of commanders and that has led to many sex assaults not being prosecuted.
"It's time to move the sole decision-making power over whether serious crimes akin to a felony go to trial from the chain of command into the hands of non-biased, professionally trained military prosecutors where it belongs," Gillibrand argued.
She pointed to a Pentagon report as evidence.
The recent study showed that there were estimated to be 26,000 incidents of sexual assault and unwanted sexual contact in 2012 and just over 10 percent of those were reported.
While Gillibrand's reforms had significant support in the Senate, they were opposed by the bipartisan leaders of the Armed Services Committee and the Pentagon.
Opponents cited concerns it would undermine the critical military principle of command authority.
"The strongest most effective approach we can take to reduce sexual assault is to hold commanders accountable for establishing and maintaining a command climate that does not tolerate sexual assault," said Sen. Carl Levin, a Michigan Democrat who chairs the armed services panel.
"The evidence shows that removing this authority from our commanders would weaken not strengthen our response to this urgent problem," he said.
Sen. Claire McCaskill, a Missouri Democrat, led the effort to defeat Gillibrand's bill.
The former prosecutor pointed to several significant reforms that became law last year she said would increase prosecutions of sex assault.
They include: removing the authority of commanders to overturn convictions; providing attorneys to victims; making it a crime to retaliate against a victim; and requiring a dishonorable discharge for anyone convicted of sexual assault.
McCaskill warned that removing commanders from the process could leave victims more vulnerable to retaliation.
Gillibrand and McCaskill, two rising Democratic senators, have battled intensely for months over how best to reform the prosecution process.
On a separate vote Thursday, a bill authored by McCaskill to make additional reforms to those adopted last year cleared a procedural hurdle in the Senate, clearing the way for expected final passage.
It would largely disallow the so-called "good soldier" defense, which permits defendants to enter evidence of their good military character at trial to mitigate the charges against them.
It moved ahead easily with the support of Gillibrand.