As the fight to stem an epidemic of sexual assault in the U.S. military reaches a showdown in the Senate this week, New York Sen. Kirsten Gillibrand unfortunately doesn't appear to have the votes to strip military commanders of the power to decide who is prosecuted or punished.

If her attempt to strengthen reforms falls short of the 60 votes needed to avoid a Senate filibuster, the burden of making sure the military lives up to its policy of zero tolerance for sexual assault will fall squarely on Defense Secretary Chuck Hagel, who blocked taking enforcement out of the chain of command.

He must demand more aggressive prosecution of sex offenders. And he has to find ways to change a military culture that is much too tolerant of rape and sexual abuse in the ranks, and so antagonistic toward victims that many don't report the crimes because they believe their attackers won't be held accountable and their own careers will be harmed.

The Pentagon estimated there were 26,000 sexual assaults in the military in 2012, up 37 percent from 2011. One in four victims said the attacker was in his or her chain of command.

The defense authorization bill the Senate will debate this week contains an alternative package of reforms sponsored by Sen. Claire McCaskill (D-Mo.). It would require civilian review when a commander overrules a military prosecutor's recommendation to take a case to court, and strip commanders of the power to dismiss sexual assault convictions. The bill also would make it a crime to retaliate against a victim who reports a criminal offense, and mandate that anyone found guilty of rape or another serious sexual assault be discharged from the military.

Those changes should help improve the pitiful conditions right now. But they would leave military commanders in control, an arrangement that has failed spectacularly for decades.

Hagel and his supporters may stymie Gillibrand's radical approach. So he must prove that he can change the culture.