Answer
Donaldson primarily opposed David Trimble within the UUP. The crux of their disagreement rested on the issue of the Good Friday Agreement, which Donaldson vehemently opposed due to its perceived concessions to Irish nationalists and the potential erosion of Northern Ireland's political and cultural identity. This opposition to the agreement became the defining characteristic of Donaldson's political stance and significantly influenced the internal dynamics of the UUP.