WASHINGTON — A full federal appeals court on Wednesday let stand an earlier ruling that President Trump’s accounting firm must turn over eight years of his financial records to Congress, bringing the case to the threshold of a likely Supreme Court battle.

In the latest of a string of court losses for Mr. Trump over his uncompromising vow to fight “all” subpoenas from Congress, the United States Court of Appeals for the District of Columbia rejected his request that it rehear a case in which he challenged the subpoena to the firm, Mazars USA. A panel of the court had sided with lawmakers in that earlier ruling.

The president will now appeal to the Supreme Court, said a lawyer for Mr. Trump, Jay Sekulow. If the justices take the case, as seems likely, it would add another blockbuster case over separation of powers to the court’s current term, which ends in June — in the middle of the presidential election campaign.

“In light of the well-reasoned dissent, we will be seeking review at the Supreme Court,” Mr. Sekulow said in a statement.