The Security Council open a showdown debate on Namibia last night by refusing a hearing to members of the territory's internal political party while agreeing to hear representatives of SWAPO, the guerrilla group seeking to wrest the territory from South Africa.

U.S. Ambassador Jeane Kirkpatrick warned that by this decision "the council defines itself and damages its capacity to act as an impartial mediator" in the dispute over Namibia, or Southwest Africa, special correspondent Michael J. Berlin reported.

Western diplomats expressed fears that the action would play into the hands of South Africa, giving it an excuse to reject the West's three-year-old plan for U.N.-supervised elections in Namibia, on grounds that the council has demonstrated bias.

Only Japan and the five Western members of the council voted to hear Peter T. Kalangula, president of the Democratic Turnhalle Alliance -- the political grouping supported by South Africa. Representatives of SWAPO were granted a hearing without opposition.

A South African spokeman here withheld comment on the council action, but said that Brand Fourie, director general of the South Africa Foreign Ministry, would make a "far-reaching" pronouncement on the matter in an address to the council today.