The battle over sexual assault in the military is back.

When the Senate reconvenes this month, members will immediately dive into a legislative struggle over how the armed services deals with accusations of sexual assault within their own ranks. And as it does, two high-profile Democrats—one a rising star in the party and the other a battle-tested moderate holding a red-state seat—will resume their internal scrap over the issue.

Both New York's Kirsten Gillibrand and Missouri's Claire McCaskill agree the military needs to change the way it handles assault cases, but they have competing proposals on how to do it.

McCaskill's camp believes that Gillibrand's plan would actually hurt the effort to fight sexual assault in the armed forces. And while they compete for their proposals, relations between the pair have become increasingly tense.

(Read about Sen. Gillibrand's proposed reforms here)

"I'm frustrated that the reforms that we have done have not gotten the attention they deserve," McCaskill said in November of her dispute with Gillibrand. "I'm not sure that I've done so well at the public relations on this; I'll give that to her."

Indeed, 2013 saw Congress take action. The Senate's final act of the year was to pass the 2014 defense authorization act, which included more than 30 reforms to address sexual assaults in the military.

But with the Department of Defense estimating 26,000 sexual assaults in the military in 2012, of which only 3,300 were reported, both legislators want to go further.

McCaskill is proposing a package of more moderate reforms that would increase commander accountability, allow survivors to challenge unfair discharge from the service, and stop soldiers from using good military character as a defense. The plan is noncontroversial, enjoys broad bipartisan support, and is expected to be adopted easily whenever it comes up for a vote.

But it doesn't include one of the most highly sought-after reforms by victim advocates: stripping the chain of command of its power to decide whether sexual assault cases are prosecuted.

The Pentagon is staunchly opposed to that proposal, but Gillibrand pushed legislation to make the change in 2013 and says she'll keep fighting for it when the Senate returns. Senate Majority Leader Harry Reid plans to put the measure on the Senate schedule as soon as January.

"We are going to have an up or down vote on the bill, probably in January," Gillibrand said in an interview before the Senate adjourned. "I will use this interim time to continue to build support. We probably have a dozen undecided senators that I'm hoping this will give an opportunity to make sure they meet with survivors from their states."

Fifty-three members are publicly supporting Gillibrand. But she is facing an uphill challenge to secure a supermajority of 60, which will likely be necessary to move the measure through the Senate.

"There is so much additional information that has been coming out in the last month that I'm hoping it provides the time to give that information to those who are considering it and weighing it," she said.

The Senate was expected to vote on both Gillibrand's and McCaskill's measures in November, but lawmakers failed to reach an agreement to do so.

McCaskill, who has long worked on sexual-assault issues and is more senior than Gillibrand on the Senate Armed Services Committee, has emerged as Gillibrand's top opponent in the reform debate. She argues that Gillibrand's measure will not result in putting more perpetrators behind bars and has taken great lengths to kill her rival's bill.

As the duo continue to duel, groups advocating for assault victims continue to push for Gillibrand's proposal.

"We're going to be doing a lot of outreach during the recess in individual states, addressing folks still undecided," said Greg Jacob, a policy director with the Service Women's Action Network and a former Marine. "We saw tremendous success before the originally scheduled vote in November with six additional cosponsors coming into the fold after bringing survivors and commanders in the field to the Hill."

Victim advocates are also focused on ensuring the reforms set to go into effect under the defense authorization legislation are implemented appropriately by the Pentagon.

Under the reforms, commanders lose the power to overturn convictions, or to reduce guilty findings to lesser offenses. Minimum sentences are established for service members found guilty of sexual assaults.

Personnel records will include sexual offenses and retaliation will become a crime.

Survivors of sexual attacks will be provided with special legal counsel, and will be allowed to apply for permanent transfers to other units. A five-year statute of limitations for rape and sexual assault in the military is thrown out. Victims will no longer be allowed to be harassed in pretrial investigations to determine whether a sexual-assault case should proceed.

With Democrats divided over further reforms, President Obama is treading lightly—pushing the Pentagon on the issue without choosing a side between Gillibrand and McCaskill.

Obama issued a statement the day after the Senate passed the sexual-assault reforms in the defense bill, warning the Pentagon, which is conducting a review due in a year, that additional reforms might be necessary.

"Members of Congress, especially Senators Gillibrand and McCaskill, have rightly called attention to the urgency of eradicating this scourge from our armed forces," Obama said in a press release.

"If I do not see the kind of progress I expect, then we will consider additional reforms that may be required to eliminate this crime from our military ranks and protect our brave service members who stand guard for us every day at home and around the world."