Mike Mansfield represented Montana in the U.S. Senate from 1953 to 1976. Initially, he endorsed U.S. policy towards Vietnam and supported Diem. However, he quickly changed his mind when Kennedy sent him in an assessment mission to Vietnam. Mansfield returned with a very negative report, warning the President against deeper involvement in Southeast Asia.

The Senator continued pressing his moderate position during the Johnson Administration. In 1965, Mansfield and Senator Russell tried to woo the President into a diplomatic offensive to negotiate a way out of Vietnam. Johnson refused.

Even though Mansfield remained one of the most active Senators in opposing the Administration's course in Vietnam, Johnson included him in some policy discussions.

Memorandum from Sen. Mansfield to President Kennedy
From the Historian of the State Department
http://www.state.gov/www/about_state/history/vol_i_1961/t.html (Doc. #207)

Senator Mansfield consistently opposed the use of American combat forces in South Vietnam. In November 2, 1961, as the coup against Diem was happening, he warned President Kennedy that "[t]he sending of American armed forces to Viet Nam may be the wrong way[...]"