Mauritius swings left - but not as far as expected

Does the sweeping victory of the leftist Mauritian Militant Movement (MMM) in the recent Mauritius general election threaten US base rights on Diego Garcia, the Indian Ocean atoll 1,200 miles to the northeast?

Before the election, many outsiders were forecasting that an MMM victory would. This view was encouraged by the pro-Western, octogenarian Mauritian prime minister, Sir Seewoosagur Ramgoolam, whose Labor Party lost all its parliamentary seats in the June 11 election.

But as the dust settles after this shattering but peaceful political upset, correspondents who covered the vote read the signs as indicating that any difficulties the new MMM government makes for Britain or the United States are more likely to be rhetorical than actual.

Diego Garcia, in the Chagos Archipelago, was administered as a dependency of Mauritius until the latter acquired independence from Britain in 1968. At that time, Britain negotiated a treaty with Prime Minister Ramgoolam under which the atoll passed under British sovereignty.

The British subsequently worked an arrangement with the United States that gave the latter base rights on Diego Garcia. The US has been turning the island into a linchpin for naval and air forces in its overall strategic plan to defend the Gulf and the Indian Ocean.

The 1,400 inhabitants of Diego Garcia have been transferred to Mauritius. As recently as March 27, Britain negotiated a $7 million trust fund to compensate them.

During the election campaign, the MMM said it would fight in every international forum to recover the ''stolen'' atoll from Britain and get the US base removed from it. The party has vowed to join other Indian Ocean states in Africa and Asia in a campaign to make the ocean a nuclear-free zone.

But with victory, the MMM is doing its utmost to give itself a moderate image abroad. It claims: ''We can best be compared with the socialist parties of France or Spain. In foreign affairs, we will be strictly nonaligned. We won't allow either Soviet or American naval vessels to call here (in Mauritius). Our closest contacts will be with France and India.''

A preference for associations with France and India may seem at first sight anomalous for a country that is a member of the (British) Commonwealth and of the Organization of African Unity. But 67 percent of the population of just under 1 million is Indo-Mauritian, 28 percent Creole, and 2 percent Franco-Mauritian. France was the colonial power in Mauritius from 1721 until 1810. In 1810, the British took over, bringing in workers from India for the island's sugar plantations - still its main source of income.

The new MMM prime minister, Aneerood Jugnauth, the party's president, is an Indo-Mauritian. The party's youthful secretary-general, Paul Berenger, a Franco-Mauritian, is the finance minister.

Mr. Berenger was at university in Paris in 1968 and was active in the student uprisings there in that year. His more conservative political foes have cited this in their efforts to establish an unfavorable image of him as a dangerous radical.

But Berenger insists that his party is committed to parliamentary democracy. The only constitutional changes he proposes at the moment are:

* Turning Mauritius into an Indian-style republic, which, like India, would remain within the Commonwealth. (At present, the island is a nominal monarchy, with the Queen as titular head of state.)

* Modifying the provision that full compensation must be promptly paid to a company that is nationalized - compensation at full market value in convertible currency. (The MMM's platform envisages nationalization of two of the island's privately owned sugar estates.)

The MMM won the 60 seats allocated to the island of Mauritius in the June 12 election. The other two seats went to a local party from the adjoining small island of Rodrigues. One of the two Rodrigues representatives has been given a ministerial post in the Jugnauth Cabinet.

Correspondents attribute the sweeping MMM victory to the desire of a youthful population (48 percent under 20 years old) for change, against a background of growing economic problems because of the slump in the world sugar market. Sir Seewoosagur Ramgoolam had been in office as chief minister and then prime minister since before most Mauritians were born.