For nearly a decade, Senator Kirsten Gillibrand, Democrat of New York, has tried to pass legislation that would remove military commanders from their role in prosecuting service members for sexual assault.

On Thursday, Ms. Gillibrand was flanked by several lawmakers from both parties to announce her latest effort, which has attracted a new and wide array of support that greatly enhances its chances of becoming law.

“We owe it to our service members to do more to prevent these crimes and prosecute them when they occur,” said Ms. Gillibrand, whose bill would require specially trained military prosecutors to decide whether or not to try assault crimes in the military, taking that decision away from commanders.

Joining her were Republican Senators Chuck Grassley and Joni Ernst, both Iowa Republicans, as well as Jeanne Shaheen, Democrat of New Hampshire, and Richard Blumenthal, Democrat of Connecticut.

“She is our leader. I try to help,” Mr. Grassley said. “If you are right, you eventually win out in the Congress of the United States. Sexual assault cannot be tolerated anywhere, but particularly in the military.”

Ms. Ernst pushed for numerous prevention efforts to be added to the bill before lending her name to the proposed legislation.

Those who bring their sexual assault accusations to commanders say they often face retaliation, and many also say that perpetrators often are not brought to justice. The number of sexual assault cases has remained high for years, according to military statistics.

“Like so many other survivors, I made the difficult decision to report what happened to me,” said Amy Marsh, a military spouse who was assaulted. She added that she and her family were repeatedly harassed.

Had there been a prosecution process outside the chain of command, Ms. Marsh said, “I might have had a shot at sharing my side of the story. My belief is that our armed forces cannot shy away from what is right.”

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.

While military leaders and chairmen of the Senate Armed Services Committee have resisted the change for decades, members of a new panel reporting to Defense Secretary Lloyd J. Austin II have made recommendations similar to the proposed legislation.