In OTL, the United Party was very close to winning. The National Party won only five seats more then the UP. Let’s say butterflies switches the number around and the United Party wins the 1948 elections.
I assume that Jan Smuts still dies in 1950. He is followed by J.G.N. Strauss. Now, what happens to the National Party? Is Apartheid dead in the water? What happens to South Africa on a whole?
There were a couple of things that might have done this.
The South African constitution had a method for adjusting constituencies. Because of the problems of travel in rural constituencies, they were given a variance. After figuring up the number of voters in a hypothetical constituency, the sizes of the various constituencies would be set such that rural ones would have only eighty percent of that figure, while urban ones would have a hundred twenty percent to compensate.
Nationalist voters were in the countryside, United (and Labour) voters in the cities. That was how the Nationalists won a small but clear majority with only
forty percent of the vote!
Smuts had been presented with a proposal for redistricting. The variance was no longer necessary, due to the spread of motor cars, the cinema, wireless, &c.; a rural parliamentarian could see everyone in his district with the same ease that his urban colleague could. But he didn't bother.
Even if he hadn't . . . .the South African Parliament then had contained four "Native Representatives", elected by the black population. (White men, of course.) A proposal had been made to increase this delegation to ten. The six extra votes would have given the United Party and its partners a small majority.