Thousands of black Salisbury residents preparing to visit country cousins this weekend could well return to the capital Monday with a political verdict eventually dooming Prime Minister Ian Smith's plan for black majority government.

The weekly interchange between blacks in the Salisbury townships and their families in the bush is a long established custom. Gradually during the five-year-old guerrilla war, the bush, where the fighting is going on, has come to call the political tune rather than the westernized townspeople as was true in the past.

Already there are visible signs that the country cousins are losing faith in Bishop Abel Muzorewa, president of the United African Nationalist Union, who generally is credited with having the largest following of any of the nationalists negotiating with Smith.

But simply, they do not understand why the bishop suddenly gave into Smith this week when grassroots supporters only days before had gathered in Salisbury to urge him to dig in his heels.

At issue was the prolonged argument over the number and nature of parliamentary seats reserved for the white minority in a country run by a black majority government. Smith got his way on the number - 28 of the 100 seats. He also made sure all would be his conservative Rhodesian Front supporters, who thus could play a major role in future black-dominated politics.

In the process, his critics say Smith may well have fatally undermined the bishop, norminally his natural ally. If Smith's victory turns out to be pyrrhic, the fault will be less any evil intent than his natural tendency to give any opponent, white or black, as little as possible.

If black disenchantment with the bishop develops into a trend, Smith may have forfeited his only reasonable chance for success in selling any so-called internal settlement to the outside world, which looks askance at his every action.

Such is the nature of black Rhodesian society that popular support for the bishop could be without any other internally based nationalist rival, or the guerrilla leadership of the patriotic Front, filling the void.

This is not a world of public opinion polls and any such movement is still virtually imperceptible, but black Rhodesians, confronted with problems of a complexity that would defy a superpower's best think tank, even now tend to retreat into silence when asked to explain their political preferences or what the future holds.

Without visible and vocal popular support for the bishop, Smith's exercise can scarcely hope to rally the international recognition required for its ultimate success.

In such matters the margin of fatal error is small, say, only 30 per cent of the vote for the bishop's candidtates rather than the 60 per cent of the vote for the bishop's candidates rather than the 60 per cent he could reasonably have hoped to win.

Moreover, in the bush where the guerrillas are active - especially so in denouncing the "sellout" Salisburt constitutional talks and all their works - the very decision to vote in an eventual election is an act of courage that lukewarm supporters of the accord might find prudent to avoid.

Vacillation and political inexperience have been the bishop's trademark, and oddly have helped explain his popularity, for he was considered by his African peers as an honest man ho, unlike his quicker-witted rivals, was not out for himself.

This week, however, his almost legendary instinct for making the right decision would appear to have deserted him, perhaps for good.

His critics now are writing his off as a perpetual bungler who never really should have been in the big leagues.

This would be to forget that the bishop still bathes in the now somewhat distant glory of having been the only African black to have stood up to Smith, and won his point. That was in 1971 when he led the successful African opposition on to a British-Rhodesian constitutional settlement.