WASHINGTON -- Sen. Kirsten Gillibrand's bill to fundamentally revamp the military justice system for sexual assault victims hit a wall Thursday when it failed to advance in a procedural vote.

An unusually bipartisan majority in the Senate voted 55-45 to break a filibuster of her bill, but that fell short of the 60 votes needed to clear it for a final vote. Gillibrand (D-N.Y.) lost two co-sponsors and couldn't win over undecided senators.

"We know the deck is stacked against victims of sexual assault in the military, and today, we saw the same in the halls of Congress," said a defiant Gillibrand.

"As painful as today's vote is, our struggle on behalf of the brave men and women who serve in our military will go on," she said. "I will never quit on them."

Gillibrand said her next step will be to try to attach her measure later this year to the annual defense authorization bill, as she tried to do last year, and to continue to push for reform.

After the vote on the Gillibrand bill, the Senate also unanimously voted to end a filibuster against a bill by Sen. Claire McCaskill (D-Mo.) that modifies the defenses that can be used in military sexual assault prosecutions, and set a final vote on it for Monday.

That bill was seen as political cover for those who voted against Gillibrand, allowing them to show they want to address sexual assault as well.

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Advocates for Gillibrand's bill expressed disappointment.

"This was a missed opportunity for Senate leaders to overhaul the military's broken system of combating sexual trauma," said Paul Rieckoff, president of Iraq and Afghanistan Veterans of America.

The vote Thursday was the latest setback for Gillibrand's yearlong drive to transform military justice by shifting decision-making authority on prosecuting sex and other crimes from the chain of command to military lawyers.

Since she held her first hearing featuring victims of sexual assault last March, she has built a bipartisan and ideologically diverse coalition, which includes 17 of the 20 women senators and Sen. Bernie Sanders (I-Vt.) on the left and Sens. Ted Cruz (R-Tex.) and Rand Paul (R-Ky.) on the right.

Her measure drove passionate debate before the vote and some bitter accusations after.

On the Senate floor, Sen. Lindsey Graham (R-S.C.) called Gillibrand's bill "social engineering run amok" and said, "This is about liberal people wanting to gut the military justice system."

Afterward, Graham complained about Gillibrand's aggressive tactics and said she won 55 votes because some senators fear being "on the wrong side of the war on women."

During the debate, McCaskill sought to downplay the rift between Gillibrand and her, saying she and Gillibrand agreed on all but one proposal.

McCaskill also said the Senate had approved "35 major reforms in less than a year" to make the U.S. military the "most victim-friendly."

Gillibrand responded coolly, saying it was not time to start "patting ourselves on the back." Instead, she said, "This problem isn't even close to being resolved."