During the 13th Congress (2004–2007), as Chairman of the Senate Committee of Constitutional Amendments and Revision of Laws, he upheld the supremacy of the Constitution at all times. Though he may not have voted for the ratification of the 1987 Constitution, he took an oath to preserve and defend it. He insisted on the lawful process of Charter Change only according to the process set forth in the Constitution. He opposed the method of Constituent Assembly or "con-ass" initiated by President Arroyo and House Speaker Jose de Venecia, Jr., which was possible in the 1935 Constitution but unlawful in the current Constitution, as well as the dubious Sigaw ng Bayan People's Initiative and was one of the triumphant parties in the case of Lambino and Aumentado vs. COMELEC, G.R. No. 174153, October 25, 2006.