Sen. Rob Portman, R-Ohio., is technically undeclared but said he's planning to support McCaskill and is probably a "no" on Gilllibrand, because of concerns he has heard from military leaders.
Sen. Sheldon Whitehouse, D-R.I., another senator who was recently considered undecided, said he is with McCaskill and Reed, the senior senator from his state.
"I've been with Claire McCaskill," Whitehouse said. When asked why he is against the Gillibrand bill, he said, "I'm supporting Jack Reed."
Other members—including a couple who remain in the undecided camp—aid they are uncomfortable with the fact that the Gillibrand bill would change the protocol of the military-justice system beyond sexual crimes.
It would cover some other crimes, considered a felony in the civilian justice system, that are punishable by a year or more in confinement—including robbery, forgery, extortion and even murder.
"I still haven't taken a position on it," said Senator Marco Rubio, R-Fla. "I still have been talking to a lot of the military leaders including many in my own state to get a better indication of how it would work…. It is a major change in the way we prosecute crimes in the military. I just want to make sure it doesn't undermine the chain of command."
The issue of encompassing a broader class of crimes than just sexual assault has so far been a deal breaker for some potential supporters, but Senate aides say that Gillibrand is not looking to further limit the scope of her bill as she had flirted with last year.
"I thought it was too broad for pulling out non-sexual-assault cases.… There hadn't been a problem in that area," said Senator Tim Kaine, D-Va., who said he is a "no" for now. "What we ought to do now is implement those [reforms included in the NDAA] and see how they work and if we haven't done enough and we need to do more we can revisit it."
The procedure for the vote has not been worked out yet. It is technically possible that 60 votes would be required to take up Gillibrand's bill, but that it could then pass with a simple majority. But Gillibrand has said she expects approval will require 60 votes.
The last time the Gillibrand measure was slated to come up for a vote, the senator enjoyed a sudden surge of momentum. She won six additional supporters, including key members like Senate Majority Leader Harry Reid and Senate Majority Whip Dick Durbin. They helped push her over the critical simple majority threshold to her current tally, in the lead up to an expected vote on her measure in the form of an amendment to the defense authorization bill late last year.
Advocates say they will need the pressure of lawmakers staring down a vote again to focus their attention and restart momentum.
"Keeping momentum, that is the challenge," said Greg Jacob, the Service Women's Action Network policy director and a former Marine.
Nancy Parrish, the president of Protect Our Defenders, said they will keep lobbying undecided and opposed senators to get 60 votes even if it takes multiple years.
"We're not going away," she said.
Gillibrand is continuing to work her colleagues. "The survivors' day on the Senate floor is coming and we will work as hard as we can until the gavel comes down to give them the justice system they deserve," she said.
"Nowhere in America does a boss get to decide whether or not a sexual assault occurred except the military. That needs to change."