Senator Bernie Moreno, Republican of Ohio, broke his silence on Sunday about a yearslong family struggle involving domestic abuse allegations against his former son-in-law, Representative Max Miller of Ohio, saying in an emotional statement that Mr. Miller was not fit to hold public office.

Mr. Moreno, saying the dispute had put his family through “pure hell,” scathingly denounced Mr. Miller, whose ex-wife, Emily Moreno, has accused him of abusing her and their 2-year-old daughter.

“If there are any basic standards of character required to hold elected office, Max Miller fails them,” Mr. Moreno wrote in a post on X. He added: “I hold my breath every minute he has custody of my granddaughter.”

He continued: “He should not serve in the House of Representatives. I believe Max Miller needs to seek professional help to end the clear pattern of abuse he has left in his wake. I believe he should not be free to continue endangering others until he does.”

The senator issued his statement in reaction to an earlier, defiant livestream on Sunday by Mr. Miller, who is seeking re-election. He denied the accusations and vowed that he was “not getting out of this race.”

Mr. Miller is running in a tight race in what had been a safe seat for Republicans before the allegations became public. Mr. Moreno’s statement, his first on the issue, seems likely to further damage Mr. Miller’s campaign, a source of concern for Republicans nationally even as they have largely maintained public silence.

The wrenching public back-and-forth came just three days before the deadline for Republicans to replace Mr. Miller on the ballot — or risk locking him in as the G.O.P. nominee for a seat that has become competitive in large part because of the allegations against him. Mr. Miller appears just one percentage point ahead of his Democratic challenger, according to internal polling research by Democrats, in a district that President Trump won by double digits in 2024.

Until now, Republicans across the board have chosen not to denounce Mr. Miller or call for his resignation, dismissing the charges against him as mere “allegations.” It was not clear on Sunday how Mr. Moreno’s weighing in would shift the response from other Republicans.