Ronald Dellums, a black, and openly socialist democratic member of the senate, began his campaign to end US support for the South African regime for apartheid in 1972.
By 1986 he had put together the Comprehensive Anti-Apartheid Bill which had the full support of both sides of the house, both Democratic and Republican.
However, the then President Ronald Regan vetoed it and then the veto was overturned by house.
Why did President Ronald Reagan exert his veto on a bill, which the subsequent history was on the right side of history?
Edit: from Wikipedia, the reason given by Reagan, was that it would mostly hurt the impoverished black majority and lead to more civil strife.