Earlier, Levin said amendment he favors, sponsored by Sen. Claire McCaskill, D-Mo., would preserve the role of commanders in the decision-making process to prosecute sexual assault cases.
"We want our commanders to be part of the process and not divorced from it," Levin said as the Senate began debate on the sweeping 2014 defense authorization bill.
But Gillibrand, D-N.Y., has the support of a majority of the Senate and a broad range of backers, from liberals to Tea Party conservatives like Sen. Ted Cruz, R-Texas, and Sen. Rand Paul, R-Ky. Reid is a huge addition to that list.
Still undecided is Paul's Kentucky Republican colleague, Minority Leader Mitch McConnell.
The Gillibrand proposal would remove the military chain of command from deciding which sexual assault cases to prosecute and instead give that authority to trained military prosecutors.
Gillibrand announced two more new supporters of her amendment, telling a Capitol Hill news conference that Sen. Dean Heller, R-Nev., and Sen. Corey Booker, D-N.J., have endorsed her approach, bringing the total to "well over half" the Senate.
But Gillibrand will need 60 votes to prevent a filibuster or block parliamentary procedures that could keep her amendment from being included in the $625 billion bill that outlines spending for the Department of Defense.
She faces opposition from a bipartisan group of senators including Levin, McCaskill and Sen. Kelly Ayotte, R-N.H.
McCaskill said her amendment would make it a crime to retaliate against a sexual assault victim and require anyone convicted of sexual assault or rape in a court-martial to be dishonorably discharged.
McCaskill and supporters of her proposals have called Gillibrand's approach a radical makeover of the military justice system.
In her news conference, Gillibrand, surrounded by retired military generals and rape victims, said that the chain of command could not objectively make a decision to prosecute subordinates.
The senator defended her amendment as the best way to achieve "accountability, justice and fairness."
Retired military brass who attended the news conference agreed.
"These improvements will remove the inherent conflict of interest that clouds the perception and, all too often, the decision-making process under the current system," said retired Army Brig. Gen. Loree Sutton.
Gillibrand's amendment also is supported by 16 of 20 women senators.
"We have to change the way the military investigates and prosecutes cases of sexual assault within its ranks," said Sen. Jeanne Shaheen, D-N.H.
A vote on the competing proposals could come as early as Wednesday.
Supporters of both approaches favor new reforms after a string of embarrassing incidents and sexual assault scandals that have marred the military justice system.
The assaults on female and male recruits, subordinates and other personnel have been widely reported and prompted numerous congressional hearings this year.
One scandal included the widespread sexual abuse of enlisted trainees at Joint Base San Antonio-Lackland, where more than a dozen Air Force instructors have been charged or implicated in allegations of misconduct.
And reports this year by the San Antonio Express-News documented repeated cases of retaliation against assault victims, failure to provide legal and medical support for them and dismissal from the service after they reported abuse.
The Pentagon estimates there were 26,000 sexual assaults in the military last year, but little more than 300 prosecutions resulted from those complaints.
Gillibrand said the low number of prosecutions underscores the need to take the decision-making process out of the chain of command.
Giving the authority to trained military prosecutors would better protect victims from retaliation and encourage them to come forward with complaints, she said.