The pro-apartheid Conservative Party extended its series of recent political victories by winning a special election Tuesday to fill a vacant seat in Parliament's white House of Assembly. Conservative candidate Corne Mulder received 8,437 votes to 4,726 for Boy Geldenhuys of the governing National Party in the balloting in rural Randfontein district. The seat became vacant Jan. 13 when Mulder's father, Connie Mulder, died of heart and kidney failure. The elder Mulder had captured the seat for the Conservatives in the national whites-only election last May. Tuesday's lopsided vote was a further indication of the Conservative Party's growing support, especially among rural whites. President P.W. Botha's National Party holds 133 seats in the whites' 178-seat House of Assembly, Conservatives 23 and the anti-apartheid Progressive Federal Party 17 with five seats held by minor parties or independents. There are three separate chambers in the white-dominated Parliament, with the mixed-race chamber having 78 seats and the Asian chamber 45. By law and custom, apartheid establishes a racially segregated society in which the 26 million blacks have no vote in national affairs. The 5 million whites control the economy and maintain separate districts, schools and health services. The Conservatives won two special parliamentary elections on March 2 in rural white districts similar to Randfontein, which is 50 miles west of Johannesburg. In the national elections in May the Conservatives won 26 percent of the votes and they are expected to gain ground on the National Party in nationwide municipal elections in October. The National Party has governed South Africa since 1948. The Conservatives oppose all concessions to the black majority and say Botha is capitulating to blacks and international pressure. Botha's party has said it seeks gradual race reform that eventually would give blacks a say in national politics, but with the whites maintaining ultimate control. The Progressive Federal Party has been losing support in recent years and did not enter a candidate in Tuesday's election.