South Africa's new constitution - admitting coloreds and Indians into very limited political partnership with the ruling white minority, but omitting the majority blacks - leaves a definite burden of proof on both the South African leadership and the Reagan administration. The governments of Prime Minister Botha and Mr. Reagan both invested heavily in the success of this week's ''reform'' referendum. They must now demonstrate that the new South African parliamentary blueprint will lead to progress for all Africans, of whatever color.
Specifically, a test of the constitution's passage will be South African inducement to pull its forces out of neighboring Namibia (South-West Africa) in a mutual phased withdrawal of Cuban and South African troops. Mr. Botha, it was said, could not move on the constitutional and Namibian fronts at the same time. Given his clear personal triumph on the referendum, and the potential broadening of his base with the at least nominal inclusion of Indians and coloreds, he will be expected to assert more leadership on Namibian withdrawal.
At the same time, the pressure now persists on the Reagan administration to prove its South African policy of ''constructive engagement'' - harping less than Jimmy Carter did on the stark human rights abridgment in a nation where 21 million blacks endure the rule of a few million whites, and stressing ''private diplomacy'' instead - will lead to action on Namibia. The Reagan administration supported the new constitution, arguing that limited partnership for two disenfranchised groups, the coloreds and Indians, was a step in the right direction toward eventual racial equality.
Ending the turmoil in Namibia, bringing greater order to the southern Africa tier of nations, would be the Reagan administration's first major foreign-policy triumph - evidence its policy of patience, emphasizing security and mercantilist interests rather than prodding on moral and legal grounds, was of greater practical benefit. So far, Reagan's policy of constructive involvement has met with disillusionment among black Africans and others concerned about race and rights. When bannings or relocations continued to occur in South Africa, the administration would profess a lack of influence. When the administration occasionally interceded, as in securing a travel permit for a South African to visit abroad, it has been seen to underscore the limitations in US leverage.