Appointed senator in 1974, Musa joined Senators Harry Courteney and Assad Shoman in persuading the party to adopt a revised constitution, one designed to increase party democracy and rank and file participation. In 1979, Musa was elected from the Fort George Division and became a member of cabinet, serving as minister of education, attorney general, and later minister for economic development. With Shoman, then minister of health, he sought to expand the principles of party democratization to government itself and, in 1982, challenged the party leadership, issuing a public declaration that the economic crisis then engulfing the region could only be solved by increasing democracy and opening participation "to strengthen and deepen democracy in Belize." The two ministers also worked successfully to gather support for Belizean independence within Latin America and the international community.
Defeated in 1984, when the PUP won only 7 of the 28 seats, Musa was elected party chairman in 1986 and worked to expand party membership and attract new leaders. He was victorious in the next general election in 1989, serving in the PUP government as foreign minister, minister of economic development, and minister of education. Musa held onto his seat in 1993, when the PUP narrowly won the popular vote but captured only 13 of 29 seats, the number having been increased to avoid the possibility of a tied House. In 1996, George Price, PUP founder and Belize's prime minister in all but two governments, stepped down as party leader. The previous year Musa had solidified his position in the party with the support of John Briceño, a young mestizo politician with a solid following in the northern districts, the base of Musa's primary rival, Florencio Marin. Musa defeated Marin in the election for party leader, becoming only the second person to fill that post.