 A member of the Senate Foreign Relations Committee, he voted for the Gulf of Tonkin Resolution in 1964 but soon became an outspoken opponent of the Vietnam War, condemning the war's escalation in 1965.In 1968, Clark was defeated for re-election to a third term by Congressman Richard Schweiker, losing by a margin of 52%-46%.His defeat is generally credited to his support of gun control, especially the 1968 Gun Control Act, and opposition to the Vietnam War.