IN 1980, Robert Mugabe promised, in terms brokered by the British, to keep Zimbabwe a multi-party democracy at least until 1990. The arrangement ended a wasting civil war and brought Zimbabwe its belated independence. But now President Mugabe, who was easily reelected the other day in his country's first elections since independence, is expressing second thoughts about his promises of 1980. The idea of a one-party state -- his party -- looks better and better to him. Most other black African countries long ago went that way.

The argument is always the same in these situations. It is that in tribal societies party politics necessarily become tribal, and therefore divisive, and that tribal strains can be managed better and the true interests of minority tribes better accommodated in a one-party structure. But let us not be carried away by a pretense of deference to another culture. This argument is pretty much nonsense. Yes, politics can go on inside a single party, but parties are the most effective mechanisms yet devised to ensure that separate interests will be heard. No, single-party rule need not lead inescapably to repression and corruption, but Africa is littered with exhibits of the costs of ruling in this style.

Mr. Mugabe represents the largest tribe in Zimbabwe, the Shona, whose numbers and undoubted favor for him ensure, fairly enough, the indefinite continuance of his rule; he has just won 63 (up from 57) of the 80 black seats in the parliament. But it enraged him that whites voted strongly for the last hurrah of his old nemesis, Ian Smith, and he has been mumbling that he would tear up "that dirty piece of paper" -- the independence constitution -- which ensures the whites 20 seats (20 times what their number alone would have earned) through the 1980s. It also troubles Mr. Mugabe to find members of the No. 2 black tribe voting faithfully for his old partner and rival in the independence struggle, Joshua Nkomo.

Zimbabwe has done pretty well since independence. Blacks have been effective and resourceful in leadership roles, and the white minority, which long contributed to the country's misery, has shaken down and focused itself on the country's growth. All this has happened under the constitution Mr. Mugabe now finds wanting.

Zimbabweans sometimes feel more burdened than blessed by the counsel of their professed friends. These friends, including Americans and British, are hoping now they will stick with democratic ways. Surely Zimbabwe will be a better place for it. Inevitably, too, the example of Zimbabwe's going back on its 1980 word will add to the load on those in South Africa who are trying to move from a white minority setup to representative rule.