On a continent where nations have increasingly been turning to socialism and one-party rule, Nigeria is engaged in drawing up a new constitution that may soon produce Africa's first American-style government.

If it works, this enormously rich country of 80 to 100 million people - already so strikingly similar to the United States in its aggressive individualism - could become a powerful advertisement for U.S. democracy throughout black Africa.

Nigeria and the United States, which already are being drawn closer by economic cooperation, would then have a special political bond. This could well develop into an unprecedented relationship between the United States of America and the first "united states" of Africa.

A constituent assembly of 231 delegates from Nigeria's 19 states has been meeting since October in the old parliament building here to design a political system dealing with the regional and ethnic forces that have so freguently torn this nation since its independence from Britain in 1960. These forces led to the assassination of three of its first five heads of state.

Most Western observers here are impressed with the assembly's performance. They expect it to finish work by the assigned deadline of October.

At that point, the military has promised to lift the ban on political activities, allow the formation of new parties, and organize elections for a new civilian government. Nigeria would then return to a civilian rule in October 1979.

Unlike Ghana, where the military's attitude toward the restoration of civilian government remains ambiguous, the 23-officer Supreme Military Council here under Lt. Gen. Olusegun Obasanjo seems sincerely determined to hand back power, according to Western diplomatic sources and many Nigerians as well. "The present group wants out," said a Western diplomat. "They are fed up with ruling the country."

"Barring all accidents, we will go back to civilian rule," remarked an editor of Nigeria's leading newspaper, the Daily Times.

The most likely "accident," according to a more cynical Westerner, is that the civilian politicians will again "screw things up" in their race for power and the military will again have to intervene in the name of national unity. "Only the military has the ability to inflict discipline on this country," commented one Westerner, noting that in 16 years of independence Nigeria has had barely five years of civilian rule.

Nigeria is the first black African state to imitate so extensively the American political system, although Liberia earlier adopted many of its attributes. The draft constitution calls for a federal government with an elected president and vice president, 450-member House of Representatives, 95-member of Senate and an independent judiciary. The president will be elected for a four-year term and not permitted to serve more than two consecutive ones, as in the United States.

The constitution probably will guarantee freedom of speech, assembly and press, as well as other civil liberties.

The 19 Nigerian states are to have American-style elected governors and assemblies. Still, the states are weaker, and their powers to be more limited, than in America.

Despite the striking similarity to U.S. structures and titles, the new system is being tailored to deal with specific Nigerian problems.

At the Philadelphia constitutional convention, the big problem was finding a compromise between the big and little among the 13 colonies and balancing their representation at the federal level. Here in Lagos, the major issue is how to avoid the domination of any one ethnic group or region so as to prevent more military coups or another bloody war of secession such as in eastern Nigeria in the late 1960s.

The present military rulers and the civilian constitutional drafting committee, under chief Frederick Rotimi Alade Williams, have come up with a number of devices they hope will force political cooperation across ethnic regional lives.

First, the ruling Supreme Military Council two years ago created seven new states, making the total of 19. In drawing the boundaries, the council carved up the three big ethnic and regional power blocs: the Yorubas of the west, the Ibo of the east and the Hausas of the north.

Now the new constitution is to push formation of broad ethnic coalitions. To be elected president, for example, a candidate must win both a majority of the total votes and at least one quarter of the votes in two-thirds of the states. The president will also have to choose at least one minister from each of the 19 states.

Every political party seeking to gain official sanction will have to show it has supporters and offices in two thirds of the states.

There are confused reports about whether Nigerians really are changing their old political habits and allegiances. Western analysts are capable of saying in practically the same breath that "the concept of north and east doesn't exist any more" but that "it would be naive to think the new parties won't have tribal bases."

These abservers seemed convinced that state politics are cutting across the traditional ethnic blocs and point to the old Hausa-dominated north as an example.

The three northern states of Sokoto, Kano and Borno are said to be "pulling the old north apart," developing their own separate identies and politics. Within Borno, there is even one group the Kanuris, who would like to carve out their own individual state. "The old unified north is badly fragmented now," remarked one analyst.

This "stateitis" is also said to be affecting the "old east, where some non-Ibo ethnic groups have also been agitating for their own states but their requests for statehood have been denied by the federal government. The new constitution will lay down difficult criteria for creating new states.

While regional politics seem to be fragmenting, ethnic-based politics still reportedly persist at the national level.

Politics within the constituent assembly, remarked one Western observer, "is essentially ethnic and regional but most issues cut across these lines' to require compromise.

Helping to break up the old politics has been a large representation of new politicans in the assembly, and the defeat of many former ones as candidates during the state electoral college elections last summer.

Some Western analysts say the most exposive remaining issue for the assembly is the allocation of revenue although the question of whether to establish a Moslem court of appeals is stirring the most emotion at the moment because it tends to set the Moslem north against the Christian south.

Although parties are not yet legal, there is already a lot of "subterranean politicking" under way. The object is to put together a winning coalition for the presidential elections. At least six "protoparties" have been identified.

At least one Nigerian commentator finds an old theme running through all the new politicking in Nigeria - a feeling that the north will again dominate the elections. Results of a voter registration campaign in January and February support this view. Of the 47.4 million who registered, the 10 states from the old northern region provided 52 per cent of the total and those from the old east and west 22 per cent each.

"The belief that the "north" is the center of power and that a leader is emerging from there has a historical base and is rooted in the experience of the nation," wrote a Daily Times commentator.

"Even though conditions today differ from what they were in the days of the first republic, we find that this belief persists and is being constantly nourished by a kind of consensus in most of the northern states that, given the imbalances that exist between the regions, the 'north' would be doomed if it did not have political power."

Western analysts tend to agree that some coalition including the north is likely to emerge. There is even talk of a north-east alliance, a strange development given the fact the 1967-70 civil war began with the slaughter of thousands of eastern Ibos by northern Moslems.

"It's a lot easier for those who tried to kill each other to form an alliance with one another than with those who stood by and watched," an observer said.