A sordid, yearslong family drama involving two powerful Republican members of Congress from Ohio and allegations of domestic abuse is threatening to put a fairly safe G.O.P. House seat at risk in an already challenging midterm election year for the party.

Representative Max Miller, a second-term Republican representing northeast Ohio whose political rise was fueled by his close relationship to President Trump, has been accused by his ex-wife, Emily Moreno, of domestic abuse against her and their 2-year-old daughter.

Ms. Moreno, the daughter of Senator Bernie Moreno, also an Ohio Republican, has said that Mr. Miller scalded her chest and stomach by throwing hot water from a skillet at her body; held a gun to her head; pushed her against a wall and hit her head; and fractured their daughter’s collarbone and bruised her shoulder.

According to court documents, their daughter at one point told Ms. Moreno, “Daddy kill you.”

Mr. Miller has not only denied it all, he has gone on offense against Ms. Moreno, making damaging accusations of his own in a series of media interviews, court filings and countersuits that have kept the family’s personal trauma in the headlines. He has called his ex-wife mentally ill, a liar, a stalker and a dangerous individual who has made him and their daughter unsafe.

The legal and public relations war has in recent weeks risen to the level of a serious concern to Mr. Trump’s political operation, as well as White House officials, who are keenly aware that every seat matters as they stare down an electoral map that shows them at risk of losing their House majority.

They worry that Mr. Miller is harming his re-election chances and Republicans’ hopes of preserving the seat, according to three people familiar with internal discussions who spoke about them on the condition of anonymity.

And Democrats, who initially had little hope of flipping the seat, are newly bullish about their chances in the district, tempered only by a worry that Republicans may intervene and replace Mr. Miller on the ballot before an Aug. 5 deadline.

Mr. Miller, however, angrily disputed the notion that his seat was in jeopardy in a series of text messages in which he threatened to block a reporter’s number for seeking comment about the matter.

“Texas is turning blue like I’m losing my seat,” he wrote in one text. “Take a hike. After I win by more than my district, don’t bother ever contacting me again.” Mr. Miller added: “You’re disgusting,” noting that his child was involved, and saying that any such story was “trash.”

In court, he has accused Ms. Moreno of making “repeated false reports” to child protective services, medical professionals, law enforcement and others. He has filed a defamation lawsuit against Ms. Moreno, saying she was running a smear campaign against him.

He has claimed that Ms. Moreno has untreated bipolar disorder, and said she is stalking him and trying to damage his reputation. He has told the court that he fears for his safety because of Ms. Moreno’s “vindictive and unpredictable behavior,” asserting that she owns at least one handgun, and claimed that their daughter’s collarbone injury was caused by Ms. Moreno’s reckless driving.

Ms. Moreno’s lawyer, Andrew Zashin, has said that she does not own a handgun, and Mr. Miller has not specified what kind or where it is. Her lawyer has also said she does not have a diagnosis of bipolar disorder. (Mr. Miller has sued Mr. Zashin.)

The recent conduct of Mr. Miller, who was once close with the president as well as Susie Wiles, the White House chief of staff, has become a source of frustration in the president’s circle, according to people familiar with their thinking. Many of them view his political problems as ones of his own making, and his to fix.

According to internal polling research by Democrats, Mr. Miller appears just one percentage point ahead of his Democratic challenger, Brian Poindexter, in a district that Mr. Trump won by double digits in 2024. The Democratic campaign arm has yet to spend any money in the district highlighting the claims against Mr. Miller, which aides said they planned to do.

“If all the allegations prove to be true,” Mr. Poindexter said in an interview, “Miller should be in jail, not worried about retaining a seat in Congress.”

Republicans close to Mr. Trump said they had viewed private polling that was also alarming for Mr. Miller’s candidacy. A spokeswoman for the party’s House campaign arm did not respond to a request for comment on the race.

A spokeswoman for Mr. Miller’s campaign, Abigail Angelos, noted that the court has continued to permit Mr. Miller joint custody of his child, undermining the credibility of what she called “outrageous accusations” against him. She said Mr. Miller would “continue to vigorously defend his reputation against false and reckless allegations designed to inflame public opinion rather than reflect reality.”

Mr. Miller is not the only House Republican facing allegations and investigations that could complicate their re-election efforts. In Florida, Representative Cory Mills was investigated by the Metropolitan Police Department in Washington last year in connection with a report of an assault on a woman. His former girlfriend was issued a protective order against him after accusing him of threatening her with revenge porn after their breakup. He has also been under investigation by the House Ethics Committee, and has vehemently denied wrongdoing.

And Representative Chuck Edwards of North Carolina is under investigation by the House Ethics Committee for allegations of sexual harassment in his office. Mr. Edwards has denied those allegations.

But Mr. Miller’s case is, perhaps, the most shocking and detailed, and involves a family drama that has ripped apart the G.O.P. delegation representing Ohio and pitted two MAGA loyalists against each other.

Mr. Moreno has remained publicly quiet about the case, which has created an awkward disconnect between both men’s political interest — keeping Mr. Miller’s seat in Republican hands — and the family feud.

A spokeswoman for Mr. Moreno, Reagan McCarthy, said that “Senator Moreno’s top priority is the safety and well-being of his daughter and granddaughter.” The spokeswoman added: “While he will not litigate these matters in the media or respond to baseless public attacks, he will do whatever it takes to ensure a just outcome for his family.”

Mr. Miller and Ms. Moreno first met while working on Marco Rubio’s presidential campaign in 2015, but rekindled their relationship years later, after Mr. Miller’s star took off when he joined the White House as director of Mr. Trump’s advance team and became close with the president.

During the first Trump term, Mr. Miller was known internally at the White House as “The Music Man,” for his role in queuing up the president’s favorite show tunes, including “Memory” from “Cats,” when he was in a foul mood.

He was the president’s loyal choice to challenge Representative Anthony Gonzalez, one of 10 House Republicans who voted to impeach him after the Jan. 6, 2021, mob attack on the Capitol.

“Max — great guy, I can tell you,” the president said at a 2021 rally in Ohio. “He’s a great guy.”

Mr. Gonzalez promptly retired instead of seeking re-election.

Ms. Moreno left campaign politics behind to create a dating app for Trump supporters. She and Mr. Miller were married at Mr. Trump’s Bedminster golf club in 2022, with Mr. Trump and close allies including Richard Grenell and Kellyanne Conway in attendance.

For a moment in time, the union looked like it would be a boon for Mr. Miller’s career. His new father-in-law, a wealthy former car dealer and hard-right Republican, donated tens of thousands of dollars to his various campaign committees. Mr. Moreno was elected to the Senate in 2024, after defeating Senator Sherrod Brown, a Democrat.

But the golden era was short-lived. The couple filed for divorce almost two years to the day after the wedding and have been stuck in a cycle of competing lawsuits revealing details of their personal lives ever since.

Ms. Moreno is not the first woman to level domestic abuse allegations against Mr. Miller. In her memoir, Stephanie Grisham, the former White House press secretary, wrote that she dated someone else in the administration who was physically abusive; she later acknowledged she had referred to Mr. Miller.

Mr. Miller denied her allegations and sued her for defamation, claiming she was trying to ruin his campaign for Congress.

But in his recent media tour, he has also managed to reopen his years-old court battle with Ms. Grisham. Earlier this month, Ms. Grisham sued Mr. Miller, claiming he broke their nondisclosure agreement, part of a 2023 settlement, in an interview and in a social media post in which he claims she fabricated the abuse.