A widely popular proposal to force sweeping changes in how the military prosecutes felony crimes is likely to be left out of this year’s defense authorization bill, according to people familiar with the matter, ending for now what advocates called watershed legislation for equal justice in favor of a competing plan that focuses more narrowly on sexual assault and related offenses.

The proposal to establish an independent authority to determine when charges should be filed for numerous crimes was spearheaded by Sen. Kirsten Gillibrand (D-N.Y.). For years, she has promoted this approach as the best way to dissociate the military justice system from the biases of commanders and ensure crime victims — particularly women and minorities — cannot be silenced or denied justice by their superiors.

Gillibrand’s legislation struck a rare sweet spot in congressional politics this year, winning the support of most Democrats and a broad coalition of Republicans, and clear majorities in the Senate and the House. But last-minute procedural setbacks, coupled with resistance from the Pentagon, have sapped the momentum it previously enjoyed.

Based on more than a dozen interviews with senior officials and advocates familiar with private negotiations on Capitol Hill, it appears the Gillibrand provision, short of a miracle, is out. Those talks involve the top Democrats and Republicans on the Senate and House Armed Services committees. The senior officials familiar with their discussions spoke on the condition of anonymity because the $768 billion defense bill, which dictates funding for the Pentagon and other defense operations, has not been finalized.

In place of Gillibrand’s legislation, the defense bill is expected to incorporate an alternative proposal that will revolutionize how the military justice system approaches cases of sexual assault and certain related crimes, and which already received the backing of three-fourths of the House as part of its version of the defense bill, officials said. Charging decisions in those cases will become the purview of an independent special victims prosecutor, reflecting recommendations the Pentagon endorsed earlier this year. The final defense bill is also likely to designate the crimes of murder and kidnapping as falling under the authority of the special prosecutor, according to officials involved in negotiations.

But for many advocates of sexual assault reform in the military, those changes fall short. And lawmakers who want to address discrimination more broadly in military prosecutions point out that the list of crimes that would fall under the special prosecutor’s purview does not include offenses for which service members who are racial minorities are disproportionately charged.

The forces behind Gillibrand’s legislation are blunt in their assessment of the baseline changes contained in the House’s legislation.

“It’s not good enough,” Sen. Joni Ernst, the Iowa Republican who has been Gillibrand’s chief partner in pressing for a more sweeping overhaul, said in an interview. “Sure, anything is better than nothing. But it doesn’t solve the problem. It doesn’t go nearly as far as we need it to.”

Gillibrand, in a statement, responded to questions about the potential demise of her legislation by emphasizing its broad backing in Congress. To excise it from the National Defense Authorization Act, or NDAA, as the larger bill is known, would be a “disservice” to military personnel, she said.

“Our reform has the bipartisan support of 66 senators and 220 House members,” Gillibrand said. “The only way it does not become law in the NDAA is if a handful of powerful men rip it out behind closed doors.”

Much of Gillibrand’s frustration is and has been directed toward Senate Armed Services Committee Chairman Jack Reed (D-R.I.), who has expressed skepticism about her approach, despite allowing it into his panel’s defense bill draft this year.

“Senator Reed is working hard to deliver the most transformative and historic reconfiguration of the U.S. military justice system in decades,” Reed’s spokesman Chip Unruh said in a statement. “He’s working with colleagues on both sides of the aisle to make our military justice system better.”

A spokeswoman for House Armed Services Committee Chairman Adam Smith (D-Wash.) declined to comment. A spokeswoman for that panel’s top Republican, Rep. Mike D. Rogers (Ala.), did not respond to a request for comment. The Senate Armed Services Committee’s top Republican, Sen. James M. Inhofe (Okla.), said in an interview that negotiations surrounding Gillibrand’s bill were too much of “a moving target” to discuss publicly.

The campaign to improve crime victims’ ability to seek remedy through the military justice system dates back more than a decade. Most of the public debate has centered around correcting how the military prosecutes cases of sexual assault in particular, given that the Pentagon’s own numbers show it is a rampant problem and there are reports that thousands of cases go unreported.

But the debate about what reforms are necessary has been complicated by differences in Congress and within the military, many of them political and generational, about how to level the playing field for women and minorities, and disagreement over what lawmakers should — and should not — demand from the Pentagon.

Defense Secretary Lloyd Austin began the year by convening an independent commission to review how the military prosecutes sexual assaults. His and President Biden’s speedy embrace of its recommendations, including that prosecutors outside the chain of command determine when such sexual assault charges should be filed, was acknowledgment that the problem needed fixing.

The congressional debate grew complicated in the past few weeks as the Senate defense bill — which contains Gillibrand’s legislation — hit what are amounting to insurmountable roadblocks. Now, there is a race against the clock as lawmakers rush to get a defense bill to Biden’s desk before the end of the year.

The breakdown has occurred over two main points: how many types of offenses should fall under the new special prosecutor’s purview and who, ultimately, is responsible for convening the courts-martial that will hear the cases at trial.

Gillibrand’s proposal puts charging decisions for nearly any crime that is not exclusive to the military and carries a potential maximum sentence of over a year in prison into the hands of a special victims prosecutor. It has the support of House Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Charles E. Schumer (D-N.Y.), who has told Pelosi that he wants to see it in the final defense legislation, though the Senate has thus fair failed to pass its version of the defense bill, according to a person familiar with their discussions.

But people involved in the negotiations said that congressional leaders are homing in on a narrower roster of crimes. There are competing explanations as to why. Those who support the more limited approach argue that renegotiating the list could threaten the balance of the entire defense bill. Those who support a more aggressive approach argue that the resistance is born of cowardice and a reluctance to stand up to pressure from the Pentagon not to push military leaders where they don’t want to go.

Pentagon spokesman John Kirby said, “There’s absolutely nothing but support from Pentagon leadership in removing the prosecution of sexual assault and sexual assault related crimes outside the chain of command and into the hands of special victims prosecutors.”

Gillibrand and her partners are upset that negotiators have embraced the House bill’s approach of letting commanders continue convening courts-martial, distrusting the House proposal’s stipulation that the special prosecutor’s “binding” charging decisions will be enough to prevent cases from being drawn out. Instead, they want to create an entirely separate courts-martial convening authority, though it would be left to the individual service chiefs — the generals and admirals who lead the Army, Navy, Air Force, Marine Corps and Coast Guardto determine what that should look like.

The dispute has exacerbated tensions within the community of lawmakers advocating for sexual assault reforms. Leading House Democrats say their version is not only sufficient but better than Gillibrand’s, in certain respects. Officials point out that only their approach makes sexual harassment a punishable crime and that, overall, it is more specific in its instruction to the Pentagon.

“Only the House bill ensures true independence by establishing an Office of the Chief Special Victim Prosecutor in each military department that would be under civilian control, not under the judge advocate generals who . . . have been quietly lobbying Congress to water down the House bill,” said Rep. Jackie Speier (D-Calif.), chief architect of that chamber’s military sexual assault legislation, who, like Gillibrand, is a longtime critic of the Pentagon’s existing approach to prosecuting sexual assaults.

Commanders, Speier added, would have “no meaningful role” in convening the courts-martial — and would be subject to discipline themselves if they don’t heed the special prosecutor’s charging decisions.

But such assurances have failed to mollify Gillibrand and many minorities in Congress, who worry that lawmakers are missing an opportunity to make necessary change.

Members of the Congressional Black Caucus, in particular, have urged Pelosi and Smith in recent days to put more crimes under the special prosecutor’s purview. At a minimum, they want to see larceny, robbery and assault charging decisions added to the list.

“Those crimes, like stalking and like kidnapping, are often associated in these special victims scenarios,” Rep. Anthony G. Brown (D-Md.) said in an interview. And, he added, “this is where you really begin going after the racial and ethnic disparity.”

Black and Brown service members tend to be disproportionately court-martialed for disputes involving alleged theft or fights compared with their White counterparts, minority lawmakers have argued. The Pentagon does not collect such data, but a separate provision in the House legislation aims to change that. Putting such cases in the hands of a special victims prosecutor, they contend, is the only way to guarantee that commanders — who are disproportionately White — do not perpetuate such bias.

Congressional Black Caucus members have the backing of sexual-assault-reform advocates, who say that it is better for the military to address prosecutorial discrimination in all forms. Otherwise, many advocates for broader reforms fear that singling out sexual assault — where the victims are disproportionately women — could lead to more institutional bias.