He was elected to the Northern Ireland Constitutional Convention in 1975 as a Vanguard member for Belfast South, and for a time he served as the party's joint deputy leader, along with the Ulster Defence Association's Glenn Barr. The party had been established by Bill Craig to oppose sharing power with Irish Nationalists, and to prevent closer ties with the Republic of Ireland; however Trimble was one of those to back Craig when the party split over Craig's proposal to allow voluntary power sharing with the SDLP.