WASHINGTON — After decades of failing to curb sexual assault in the armed forces, lawmakers and Pentagon leaders are poised to make major changes in military laws that many experts have long argued stand in the way of justice.

A bill championed by Senator Kirsten Gillibrand, Democrat of New York, would remove military commanders from a role in prosecuting service members for sexual assault and has gained support from scores of key members of Congress. Among them is Senator Joni Ernst, Republican of Iowa and a retired National Guard lieutenant colonel, who said her own experience with assault and her daughter’s stories from West Point helped shift her views on the issue.

“I have been torn,” Ms. Ernst said in an interview. “On the one hand, I was a commander in the National Guard and know how important that role is. But also, as a sexual assault survivor, I know we have to do more. I never really wanted to take this out of chain of command, but we are not seeing a difference.”

Ms. Ernst’s nod on a new bipartisan measure is likely to attract several other key lawmakers, whose combined support could usher in the biggest change to military rules since the repeal of the ban on service by gays and lesbians in 2010. Other senators — many of whom voted against the measure in the past — said in interviews that they had waited long enough for the military to solve the problem and agreed that Congress should step in.

Lloyd J. Austin III has made a similar recommendation, saying that independent judge advocates should take over the role that commanders currently play. These independent military lawyers would report to a special victims prosecutor, who would decide whether to court-martial those accused of sexual assault, sexual harassment or domestic violence. The responsibilities could also extend to those accused of hate crimes.

The change to military law would require an act of Congress.

While Mr. Austin has said he wants service chiefs to review the recommendations, he has made clear that he is open to such a change, as has Kathleen Hicks, the deputy defense secretary and the first woman to serve in the No. 2 role at the Pentagon. A report out of Fort Hood, Texas, last year that detailed a culture of harassment and abuse has further cemented views at the Pentagon and on Capitol Hill that broad changes are needed.

The efforts reflect generational changes in the armed services, with younger members speaking out more forcefully about these issues. Just as important are the shifting views among lawmakers who have tired of ugly stories about assault of women in the military and the arrival of a defense secretary who is eager to assert his influence on an issue that vexed him during his years as an Army general.

“The tide has turned,” said Eugene R. Fidell, a senior research scholar at Yale Law School and an expert on military law. “Where the Department of Defense can be the most useful and constructive now is in helping shape the change.”

issue is not particularly partisan, unlike many major national security policy issues, though historically more Democrats than Republicans have supported the idea of taking away commanders’ control of these cases.

Senator Mitch McConnell of Kentucky, the Republican leader, has supported the bill, which last received a vote in 2014. Senator Ted Cruz, Republican of Texas and a Trump ally, is a co-sponsor of the bill. Senators Tim Kaine and Mark Warner, both Democrats of Virginia — a state with a large military population — previously opposed the measure, but they say their views are changing.

Mr. Warner said he would co-sponsor the bill, and Mr. Kaine appears to be shifting in its favor.

“I have supported virtually every change within the chain of command that we can think of to address this problem, and I’ve not been happy with the results,” Mr. Kaine said. “I don’t see the palpable difference I was hoping for. So that leaves me very open to her proposal.”

At a news conference scheduled for Thursday, Ms. Gillibrand is expected to announced her new compromise with Ms. Ernst, who has pushed for several additional components aimed at preventing assault, such as cameras in common areas and better training from the earliest entry points to the military.

“I have long said that by the time we have a survivor and a predator, we have failed,” Ms. Ernst said. “We’ve got to do more on prevention, and Kirsten agreed.”

confirmed by the Senate, Mr. Austin made sexual assault one of his first priorities. In February, he appointed an independent commission to examine the issue and give recommendations that he and the service chiefs could consider. While the department could make changes to policies, a move to take prosecution and courts-martial of assault cases from commanders to a civilian-led office of a special victims prosecutor would require an act of Congress.

The members of the panel are seeking a new career track in the Defense Department in which judge advocates general — military lawyers — would be specially trained to deal with such cases.

While sexual harassment is often a precursor to assault, the panel recommended that some cases be treated differently. If a finding of harassment is substantiated, the panel suggested, mandatory separation from the military would be initiated. In some cases, low-level misconduct by a service member who is of lower rank than the colleague they harassed could be offered something else, like a training intervention program. Among many other recommendations, the panel seeks an expansion of military protective orders off base.

While Congress and the military have approved scores of changes aimed at making it harder for those who harass and assault their peers to remain in the military, prevention efforts appear to have not yielded success so far, a focus of Ms. Ernst’s. The Air Force is beginning a program that uses virtual reality to work with airmen on how to respond as a bystander.

In 2019, the Defense Department found that there were 7,825 reports of sexual assault involving service members as victims, a 3 percent increase from 2018. The conviction rate for cases was unchanged from 2018 to 2019; 7 percent of cases that the command took action on resulted in conviction, the lowest rate since the department began reporting in 2010.

Sexual assault causes mental health issues among female veterans, and many military experts — including those on the independent panel — say that fear of harassment and worse will negatively affect retention of women in the armed services.