The senators called for Mr. Blackmun to step down after a report in The Wall Street Journal that the U.S.O.C. did not intervene despite learning in 2015 from U.S.A. Gymnastics that gymnasts were victims of possible sexual misconduct by Dr. Nassar, a year before accusations became public in an investigation by The Indianapolis Star.

The U.S.O.C. has said it followed proper procedures and was told that the authorities were being contacted. But The New York Times has reported that as an F.B.I. investigation plodded along between July 2015 and September 2016, at least 40 girls and women say Dr. Nassar molested them during that period.

The Olympic reform group has sent a 13-page report to the House Committee on Energy and Commerce, which is investigating the sexual abuse of gymnasts.

In its letter, the reform group described what it said was a failure by Mr. Blackmun and the Olympic committee to sufficiently protect athletes from sexual abuse over the past three decades in various sports such as gymnastics, taekwondo, swimming and speedskating.

Athletes who complained of sexual abuse were left to fend for themselves in arbitration instead of being able to rely on the U.S.O.C. for safety, the group said in its report. As a result, it said, Mr. Blackmun and the Olympic committee “created the underlying conditions for sexual abuse to thrive by cutting athletes off from institutional support.”

One of the reform group members is Nancy Hogshead-Makar, a three-time gold medalist in swimming at the 1984 Summer Olympics in Los Angeles and a lawyer who said she was serving as an expert on sexual abuse on behalf of some gymnasts in civil litigation.

In a statement, Ms. Hogshead-Makar said that Mr. Blackmun “allowed one of the country’s largest youth-serving organizations to avoid basic child protection measures, such as prohibitions on adults being alone with children.”