The comments from the lawyer, Ronald P. Fischetti, came days after a legal opinion was issued by the Justice Department that said that the Treasury Department must turn over six years of the former president’s tax returns to congressional investigators.

Mr. Fischetti on Monday called the opinion “absolutely ridiculous” and said that he expected the legal battle over the release of the returns to go on for months.

“We’re going to fight this tooth and nail,” he said.

The opinion, which the Treasury said that it would comply with, opened the possibility that the returns, which Mr. Trump has fought for years to keep secret, could be released to Congress. But even if the returns are handed over to Congress, Mr. Trump’s tax information may not become public immediately or at all.