South Africa President Pieter W. Botha, recovering from a stroke, resigned as head of the ruling white National Party Thursday but will remain head of state, a party spokesman said. In a letter delivered to a National Party caucus meeting at the Cape Town parliament, Botha asked party managers to separate the traditionally overlapping posts of party and government leader, spokesman Boet Botma said. Botha has been party leader for 10 years. National Party lawmakers who form the caucus elected National Education Minister Frederik de Klerk as new party leader. De Klerk is widely deemed the most likely candidate to succeed Botha as president.