The Senate on Tuesday blocked a years-long push to reform how the military prosecutes sexual assault cases.

Senators voted 50-49 on the proposal, spearheaded by Sen. Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE, which would remove military sexual assault cases from the chain of command.

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The New York Democrat needed 60 votes to add her legislation to the National Defense Authorization Act (NDAA), currently before the Senate.

Gillibrand criticized President Obama, suggesting that he hasn't done enough to tackle the issue as commander in chief.

"He promised he would address this issue," Gillibrand said. "I expect more, and I expect more leadership."

It’s the second time in two years the measure hasn’t moved forward in the Senate.

The Senate voted 55-45 last year against advancing the measure when the Democratic Party held the Senate majority.

In both instances, Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.) helped lead opposition to Gillibrand’s measure.

The Senate passed legislation from McCaskill last year that increased protections for victims of military sexual assault.

McCaskill argued on Tuesday that past reforms are making a positive change.

“These victims are coming forward because they have renewed confidence that they will have support, that they will get good information and that the system is not stacked against them,” she said.

But Gillibrand suggested that previous legislation hadn't changed the level of retaliation faced by sexual assault victims when they come forward.

“This culture must change, and if Congress doesn't take their responsibility to hold the Department of Defense accountable, no one will,” she said.

"Change is coming. It's coming through Congress, and a great many of our colleagues say privately they're very, very close to supporting this," Cruz, who is running for president, said. "Change is coming in January 2017 when a new commander in chief will be sworn into office."

Gillibrand added that she believes the legislation is relevant to the presidential election because "it's an issue of our role as oversight."

Supporters of the bill had rallied around Gillibrand's proposal ahead of the vote. Sen. Chris Coons Christopher (Chris) Andrew CoonsMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Del.) said that Congress needs “to make meaningful change to how sexual assault is handled in the military so that we can fulfill our sacred duty of protecting men and women in uniform as well as they protect us."

Sen. Chuck Grassley Charles (Chuck) Ernest GrassleyGOP set to release controversial Biden report McConnell locks down key GOP votes in Supreme Court fight Senate Republicans face tough decision on replacing Ginsburg MORE (R-Iowa), who also supported the proposal, added, “I'm not one to advocate for major, sweeping reform if less will address the problem, but what we've been doing has not worked.”

But Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.), chairman of the Armed Services Committee, said ahead of the vote that while he has “respect and appreciation for Sen. Gillibrand's passion, and for her dedication on this issue, I respectfully disagree.”

Outside advocacy groups who supported Gillibrand's position quickly slammed the Senate's vote.

Don Christensen, president of advocacy group Protect Our Defenders and a former chief prosecutor for the Air Force, said senators failing to hold the Pentagon accountable.

“Sadly, there are still many Senators willing to give the Pentagon a pass despite decades of empty promises and phantom progress," he said in a statement. "Those opposed to a fair justice system for our troops and their families are listening to the same generals that were against gay Americans serving their country or allowing women to serve equally."

Iraq and Afghanistan Veterans of America CEO and founder Paul Rieckhoff separately said that "we are frustrated and disappointed" by the vote.

It wasn't immediately clear what Gillibrand's next step would be with the legislation.

This story was updated at 5:21 p.m.