The sister-in-law, Danielle Hegseth, was formerly married to the nominee’s brother, and in an affidavit obtained by The Associated Press she said she believes that Pete Hegseth is “unfit” to run the Defence Department based on what she witnessed and heard.

She said she first relayed her allegations to the FBI in December but was concerned that the information was not shared with Congress as senators consider Hegseth’s nomination from Donald Trump to lead the Pentagon.

The affidavit describes Hegseth’s treatment of his second wife, Samantha, and alleges repeat drunkenness and a domestic situation where Samantha Hegseth had a safe word to indicate if she was in danger at home. Danielle Hegseth said Samantha texted that safe word to her sometime in 2015 or 2016, which prompted her to call a third party for help.

“I have chosen to come forward publicly, at significant personal sacrifice, because I am deeply concerned by what Hegseth’s confirmation would mean for our military and our country,” she said.

An attorney for Pete Hegseth vehemently denied the allegations. Tim Parlatore said the affidavit was filled with “belated claims” by an ex-relative with “an axe to grind against the entire Hegseth family”.

“Sam has never alleged that there was any abuse, she signed court documents acknowledging that there was no abuse and recently reaffirmed the same during her FBI interview,” Parlatore said in a statement.

“Belated claims by Danielle Dietrich, an anti-Trump far-left Democrat who is divorced from Mr Hegseth’s brother and never got along with the Hegseth family, do nothing to change that.”

The affidavit was filed in response to a request from Senator Jack Reed of Rhode Island, the top Democrat on the Senate Armed Services Committee, who said he was seeking her statement to gain “personal knowledge about Mr Hegseth’s fitness to occupy this important position”.

In the affidavit, Danielle Hegseth says she believes Hegseth’s marriage to Samantha was abusive. She said Samantha in 2014 told her she once hid in her closet because she feared for her safety. She acknowledged she did “not personally witness physical or sexual abuse” by Pete Hegseth toward his wife.

“I believe what Samantha told me because what she told me is consistent with what I personally observed of Hegseth’s erratic and aggressive behaviour over many years,” she said.

Neither Samantha Hegseth nor an attorney who represented her in divorce proceedings immediately responded to requests for comment. A lawyer for Danielle Hegseth noted that she wouldn’t be commenting further than the affidavit.

In a statement to NBC News, which first reported on the affidavit, Samantha Hegseth said there “was no physical abuse in my marriage. This is the only further statement I will make to you, I have let you know that I am not speaking and will not speak on my marriage to Pete. Please respect this decision.”

Samantha Hegseth and Pete Hegseth both signed a Minnesota court document in 2021 during their divorce saying neither claimed to be a victim of domestic abuse.

The sworn statement arrived just one day after Hegseth’s nomination was advanced out of the Senate Armed Services Committee despite initial scepticism from many Republicans. A final vote confirming him as defence secretary could come this week.

Republican leaders said they doubted the new allegations would change the votes of support they are lining up for Hegseth.

“Looks like a desperate ploy by Democrats who are being driven by the far left because they know that we have the votes to confirm,” said Senator John Barrasso, the No. 2 GOP leader.

Asked if the new allegations changed the votes for Hegseth, he said: “No.”

Senator John Cornyn said he was aware of the reported allegations and compared the situation to the claims made against Brett Kavanaugh during his own confirmation hearing, before he was eventually confirmed as a Supreme Court justice.

“I don’t really have anything to offer,” Cornyn said. “I was involved in the Kavanaugh hearings, where people came out of the closet making all sorts of false allegations.”

Questions were raised before Hegseth’s hearing last week about the scope of the FBI background check, which some said failed to include interviews with those who had raised allegations against him. Democrats pushed for a deeper FBI review that could be more broadly shared with senators.

During the hearing, Hegseth did not directly answer yes or no to questions about his drinking, instead saying the issue was part of an orchestrated smear campaign against him.

At the time, it was left to the Trump transition team to decide how much more information to pursue and share on Hegseth.

The information in the affidavit was not relayed to the leaders of the Armed Services Committee when FBI officials briefed them earlier this month on the results of Hegseth’s background check, according to a person familiar with the contents of the briefing who was granted anonymity to discuss it.

“As I have said for months, the reports of Mr Hegseth’s history of alleged sexual assault, alcohol abuse, and public misconduct necessitate an exhaustive background investigation,” Reed said in a statement on Tuesday.

“I have been concerned that the background check process has been inadequate, and this affidavit confirms my fears.”

Hegseth was grilled by senators during his hearing about his behaviour, including excessive drinking, extramarital affairs and allegations of sexual assault, which he has denied.

Hegseth has promised not to drink on the job and has denied a 2017 sex assault allegation but acknowledged paying the woman a settlement. He was going through a divorce at the time after having a child with a Fox News producer who became his current wife, according to court records and his social media posts.

In the affidavit, Danielle Hegseth also alleges that Pete Hegseth, while under the influence of alcohol and both were leaving a bar, repeatedly shouted “no means yes!”

“I took this to mean that, in his opinion, nonconsensual sex is okay,” Danielle Hegseth said in the affidavit.