JOHANNESBURG, South Africa (AP) _ The state-run television network faces charges of biased political coverage as the governing National Party heads toward what could be its most difficult election since taking power in 1948.

The South African Broadcasting Corp.'s telecasts are the main source of news for more than half of the country's 5 million whites, according to surveys.

The SABC insists it provides impartial reporting, but critics charge that a strong pro-government bias undercuts both white opposition parties and black anti-apartheid groups.

The Sept. 6 election, although excluding the black majority, is viewed as a potentially crucial event. Opinion polls suggest that the far-right Conservative Party and liberal Democratic Party could gain enough ground to deprive the National Party of an outright majority in Parliament's dominant white chamber.

The SABC has been carrying full-fledged political debates for the first time, allowing opposition candidates to skirmish with their Nationalist rivals. The debates have been welcomed, but accusations persist that SABC's news division promotes National Party interests despite a clause in the corporation's charter calling for impartiality.

''The SABC goes far beyond simply being a propaganda vehicle,'' said John van Zyl, a professor of film and television at the University of the Witwatersrand. ''It constructs a political map which shows the National Party in the middle, where the Conservative Party and Democratic Party are in disarray and the extra-parliamentary opposition groups are suppressed completely.''

Van Zyl and some of his students are conducting a study of the SABC's political coverage during the election campaign. An interim report issued Aug. 7 said the SABC had ignored the views of major anti-apartheid organizations and reported their actions in ways that were favorable to the government.

They cited as an example the protest Aug. 2 in which black and Indian patients sought treatment at white hospitals. SABC news reports suggested that police vigilance helped avert chaos and made no mention of organizers' statements that the protest was intended from the start to be peaceful and non-disruptive.

A wide range of prominent opposition leaders, including white human rights activists, black trade union officials and clergymen such as the Anglican Archbishop Desmond Tutu, are seldom heard on SABC newscasts, while routine speeches by Cabinet ministers often are reported at length.

The director of SABC news, Carel van der Merwe, said the corporation's policy is to report on actions and statements from non-parliamentary opposition groups if these are deemed newsworthy.

He said the SABC did not seek to stereotype any political parties and pursued coverage that would ''supply knowledge and insight to the viewer.''

SABC crews sometimes have exclusive access to police operations. So negative is the SABC's image among activists that its crews occasionally are evicted from anti-apartheid gatherings.

The Congress of South African Trade Unions, a militantly anti-apartheid labor federation, said it felt flattered by ''the constant barrage of lies and misinformation being spread on SABC television, radio and other National Party propaganda organs.''

Van Zyl said government officials sometimes give explicit instructions to the SABC regarding the content of news reports. One such incident in 1987 was widely publicized, when President P.W. Botha telephoned in the midst of an evening newscast and successfully demanded a revised report on his ouster of a mixed-race Cabinet minister.

Asked about government interference in the news, van der Merwe said: ''I'm not responding to dead horses.''

The SABC's television service is relatively new, dating back to 1976. It operates two white-oriented channels and two black-oriented channels.

The only independent television service is provided by M-Net, a pay-TV company established in 1986. It is barred from broadcasting news programs.

The SABC also operates more than 20 radio stations. The one independent station, Radio 702 in Johannesburg, is allowed to broadcast news and recently expanded its news programming.

The SABC's directors are presidential appointees, and most of its top managers are Afrikaners, the mainly Dutch-descended whites who dominate the National Party. But many of the corporation's 7,000 employees are blacks seeking technical experience in broadcasting that is not available elsewhere.

Budget deficits have plagued the SABC in recent years, and critics say the quality of locally produced programs has dropped. British entertainment unions won't allow their shows to air in South Africa, and most current programs are series produced a year or more ago in the United States.

Against this background, the political debates have been praised by local television critics as a small step toward meaningful programming.

''Now that the era of two-man (white) debate has arrived, perhaps the possibility will one day arise of real debate involving all South Africans,'' wrote Sheryl Raine of Johannesburg's Sunday Star. ''But don't hold your breath.''