As one of three Republicans on the Senate Rules and Administration Committee, Cooper was involved with the investigation of Johnson aide Bobby Baker in 1964, which he decried as "a whitewash" after the committee blocked further investigation. He proposed the establishment of a Senate Select Committee on Standards and Conduct in July 1964 and was named to that committee in July 1965. Also in 1965, he was chosen advisor to the United States delegation to the Manila Conference that established the Asian Development Bank.