During 1967 the popular sentiment in the U.S. was anti-war, which prompted Muskie to visit Vietnam to inform his political stance in 1968. Prior to his visiting the country, he debated with a congressman on a pro-war platform. After the trip, he became a leading voice for the movement and entered into the ongoing debate by speaking at the year's Democratic Convention. His speech was followed by "tens of thousands of protestors surrounded the convention and violent clashes with police carried on for five days." He wrote to Johnson personally asserting his position on the Vietnam War. He made the case that the U.S. ought to withdraw from Vietnam as quickly as possible. Months later, he wrote to the president again urging him to end the bombing of North Vietnam. During the same year, he traveled with other Senators to the Republic of South Vietnam to validate their elections.