A New York congressman under federal indictment, George Santos, will continue fighting to keep the identities of his bond providers a secret, a key question for those following the legal turmoil.

The judge handling the case had ruled that Mr. Santos must divulge the identities of the three individuals who provided his $500,000 bond, or appeal the decision by Friday. The embattled congressman filed a 6-page appeal shortly before the noon deadline, pushing back against the judge’s ruling.

In the appeal, Mr. Santos’s defense team argued that disclosing the identities of the bond providers will force them to withdraw their support. This would deprive Mr. Santos of the right to a reasonable bond guaranteed by the Eighth Amendment, his lawyer said.

The defense also argued that the bond providers would be exposed to “attacks and harassment” if their names are made public and that evidence has indicated the individuals are non-partisan. News organizations had said that it was in the public interest to reveal the individuals’ identities.

Mr. Santos has “has essentially publicly” said that the bond providers “are family members and not lobbyists, donors or others seeking to exert influence over the Defendant,” said the lawyer, Joseph W. Murray.

The three individuals backed Mr. Santos when 13 charges were filed against him in a federal court on Long Island last month. News organizations demanded the court disclose the names of the bond supporters, known as sureties, in a letter to the Eastern District of New York late last month, citing public interest in a case against a congressman and a legal right to access judicial records.

“There is a compelling public interest in maintaining the greatest transparency possible in these proceedings,” the news organizations said. “The public has a right of access to the identities of the sureties.”

Prosecutors said they took no position on the matter.

Mr. Santos’s attorney argued against disclosing the identities earlier this week, telling the court the individuals would likely “suffer great distress, may lose their jobs and God forbid, may suffer physical injury.”

To back up his claim, Mr. Murray sent the court threats he had received for defending Mr. Santos, including a profane voice message saying the case would “ruin your law firm.”

Mr. Murray said the congressman “would rather surrender to pretrial detainment than allow the disclosure of his benefactors’ identities.”

A magistrate judge, Anne Shields, ruled against Mr. Santos on Tuesday. She kept her decision sealed from public view to maintain the anonymity of the bond providers ahead of the appeal deadline.

The bond arrangement has also raised further ethical concerns for Mr. Santos. The House Ethics Committee last month demanded information from the congressman, including the identity of the sureties, to determine whether the bond constituted an improper gift.

The $500,000 bond was unsecured, meaning the sponsors did not have to provide the money to the court, but may be required to pay the full amount if Mr. Santos fails to adhere to his release conditions.

Mr. Santos was arrested last month on federal charges for alleged fraud, money laundering, theft of public funds, and false statements. He pleaded not guilty and decried the charges as a “witch hunt.”

After he took office last year, media reports exposed a litany of lies Mr. Santos had told the public about his background, including fabrications about his education, work history, family members, and Jewish identity.

Mr. Santos has vowed to continue with his re-election campaign. His next court appearance is scheduled for June 30.