16 April 1958
The last general election of the Union of South Africa before it became a Republic on 31 May 1961, was held with the National Party (NP) under Adv J.G. Strijdom, registering a victory. The NP managed to secure 103 seats, its greatest electoral victory up to that time, while the seats of the United Party, though under the new leadership of Adv. J.G.N. Strauss, decreased from 57 to 53.  
References

Wallis, F. (2000). Nuusdagboek: feite en fratse oor 1000 jaar, Kaapstad: Human & Rousseau. Muller, C.F.J. (ed)(1981). Five Hundred years: a history of South Africa; 3rd rev. ed., Pretoria: Academica, p. 475.|Horrel, M., A Survey of Race Relations in South Africa, from Nelson Mandela Centre for Memory, [online], Available at www.nelsonmandela.org [Accessed: 16 April 2014]