Once the starting point of Africa's early explorers, once the slave trading center for "the dark continent," once an exotic Arab stronghold, the East African island of Zanzibar is today an almost forgotten haven.

The only things the histroical isle is famous for now are the fragrant clove spices it produces for most of the world, the prized antique wooden doors with brass spikes originally designed to ward off attacking elephants, and the color television system that was Africa's first.

Yet scenic Zanzibar, just half the size of Rhode Island, may soon emerge again as an important political and economic power within East Africa.

Diplomats and insiders on the lush little island and on the Tanzanian mainland, about the size of Texas and New Mexico combined, believe Zanzibar's political hierarchy is likely to take power soon when President Juilus Nyerere steps down, a dramatic step which has been predicted for over six months.

The first step was completed in early February, when the separate parties of the mainland and the island merged. As a Western diplomat explained:

"In practical terms, there was no reason for a merger except as a preliminary step toward [Zanzibari leader and Tanzanian First Vice President Aboud] Jumbe taking over from Nyereere."

Two weeks later, there was a major shakeup in the Tanzanian Cabinet when two Zanzibaris were installed in prominent positions - as minister of state in the president's office and minister of state in the first vice president's office. This move was also viewed as preliminary to a Jumbe presidency.

Since their union as Tanzania in 1964, the former German colony known as Tanganyika and the ex-Arab sultanate of Zanzibar - just 20 miles apart - have been one in name only. Politically, economically an culturally, they have maintained their separate identities and systems.

Zanzibar even has refused to share its comparative riches in foreign reserves - held in London - with the poor mainland. The Jumbe government has also gone its own way in development projects, ignoring Nyerere's "ujamaa" specialist experiment of communal farms in favor of a more capitalistic approach.

The strength of Zanzibar's economy - responsible for a population of only 400,000 in comparison with Tanzania's 16 million - is the other reason for increasing influence.

While Tanzania's economy has suffered repeated setbacks from poor harvests and partial rejection of "ujamaas" by rural peasants, Zanzibar has pushed ahead with new low-income apartment complexes, tourist facilities, the sophisticated television system and agricultural diversificaltion projects - financed with the $50 million annual income from clove crops.

A foreign economist based in Dar es Salaam said the mainland government may be hoping that closer political ties will help bail Tanzania out of its heavy debts, which were significantly worsened by the recent collapse of the three-nation East African Community.

The lush, beach-fringed island is not problem-free. Rationing of sugar, rice, flour and other staples - one pound of each per family per week - was introduced because of chronic shortages. The government refused to use reserves of imports at prices that would not bring in profits.

The situation is better, however, on the mainland, where dairy products and other staples are often lacking and foreign currency is absent.

The island's comparative economic strength appears to be pushing Zanzibar back into political prominence, with full Tanzanian backing.

Until mid-1976, Jumbe was a relative unknown in the vast rural areas of the mainland outside Dar es Salaam. But since recent speeches and counntrywide junkets organized by the mainland party the portly Zanzibar leader is a familiar and popular figure. This also makes Tanzanians feel closer to the island.