In a statement that mapped out the legal battlefield ahead, William Consovoy said the request, formally filed on Wednesday by U.S. House of Representatives tax committee Chairman Richard Neal, flouts "constitutional constraints."
"The requests for his private tax information are not consistent with governing law, do not advance any proper legislative purpose, and threaten to interfere with the ordinary conduct of audits," Consovoy said.
"We are confident that this misguided attempt to politicize the administration of the tax laws will not succeed," he said.
One of the many investigations targeting Trump on Capitol Hill and in the U.S. court system, the House Democrat's probe into the president's tax returns could pull back the curtain on his business empire and his reputation as a savvy dealmaker.
Unlike previous presidents over recent decades, Trump has refused to make public past tax returns, while retaining ownership in many enterprises, ranging from golf courses and hotels to Trump Tower in New York City and his Mar-a-Lago private club in Florida.