Hertzog led the party till 1939. In that year, Hertzog refused to commit South Africa to Britain's war effort against Nazi Germany, as many South African Afrikaaners, Hertzog's own ethnic group and electoral power base, were of German descent and sympathies. The majority of the United Party caucus were of a different mind, however, and Hertzog resigned. Jan Smuts succeeded him and led the party and the country throughout World War II and the immediate post-war years.
Smuts and the United Party lost the 1948 election to the National Party. It was never to hold power again. Sir de Villiers Graaf replaced Smuts as party leader, a post he held until 1977. Attrition characterized his leadership years, as the party slowly declined because of electoral gerrymandering, changes to South Africa's voting laws (including the removal of the Coloureds - South Africans of mixed ancestry, who had been staunch United Party supporters - from the electoral rolls, and defections to other parties. In 1977, the United Party was renamed the New Republic Party, but a significant number of its parliamentarians refused to remain with the renamed party; some joined the anti-apartheid Progressive Federal Party and others eventually joined the ruling National Party. Elections in late 1977 left the New Republic Party gutted, with only 10 parliamentary seats, down from the 41 the United Party had held previously.
Although more moderate than its National Party opponents, the United Party was committed to White supremacy.