President Shehu Shagari's recent landslide victory has changed dramatically the face of Nigerian politics, but the numerous cases of vote-rigging and violence have left a lingering discordant note for the future of the country's domestic harmony.

Shagari's reelection was virtually unchallengeable because of his more than 4 million-vote victory margin over his nearest competitor and his ability to score deep inroads in the regional ethnic bases of is two main rivals.

The elections underscored Nigeria's commitment to civilian democracy. But marring Shagari's win and the subsequent elections for state governorships, state assemblies and both houses of the National Assembly was the violence and the often obvious falsification of election results by agents of all six parties in collusion with local officials of the Federal Electoral Commission.

Shagari's National Party of Nigeria has borne the brunt of the harshest allegations of using its incumbent position to substantially expand the victory at all levels of the government. Independent western sources here said Shagari would have probably won anyway without that help but not with such a wide margin.

The polling malpractices sparked rioting and killing in three states--in a number of instances with victims being burned alive--before the Aug. 6 to Sept. 3 set of five elections ended.

With an estimated 100 million population and as black Africa's largest and richest oil exporter, what happens in Nigeria has a wide impact on this continent. One of every four Africans is estimated to be a Nigerian. The country's four-year fledgling effort at multi-party democracy, therefore, is seen as eventually influencing other African governments, most of which are one-party states or military dictatorships.

Shagari was first elected in 1979 and took control from a military government that had ruled Nigeria through 13 years of turmoil. The Army overthrew the country's first civilian government in 1966, six years after independence from Britain, after vote-rigging by politicians and government officials sparked rioting and a breakdown in law and order among the Yoruba people in southwest Nigeria.

That first coup led to a countercoup, then a tragic, 2 1/2-year civil war, a third coup and a bloody attempted coup before Nigeria returned to civilian government.

In this election, Shagari, 58, proved to be more popular than his National Party of Nigeria. A lot of politicians running under his National Party's banner benefited from "a bandwagon" effect that followed in the four elections after Shagari's victory, according to western diplomatic sources. Shagari's National Party members now hold the governor's seat in 11 of 19 states (up from seven) and have absolute majorities in both houses of the National Assembly (up from less than half in each).

Shagari's successes against his two main rivals, Obafemi Awolowo, 74, and Nnamdi Azikiwe, 78, have helped him emerge as a truly national leader despite the Moslem Hausa-Fulani heritage of his northern Nigeria homeland. Awolowo has been reduced to a regional Yoruba leader, and Azikiwe, Nigeria's first elected president, has been reduced to an even smaller role, comparatively, among his Ibo people in eastern Nigeria.

Shagari's low-key and humble style of governing in his first four-year administration clearly met with widespread approval among the Nigerian electorate. Among a field of six candidates, Shagari polled 48 percent or 12 million votes of the 25.79 million votes cast. His nearest rival Awolowo received 7.8 million votes and most of that came from the densely populated Yoruba states.

Together, the Hausa-Fulani, the Yoruba and the Ibo make up 60 percent of Nigeria's population. The rest of the population is formed from about 250 ethnic minorities fearful of dominance by any of the big three. Shagari, however, clearly received their vote of confidence.

Shagari's National Party's sweep of the seats in the governors' races and the National Assembly means that his party is now a national party and no longer a vehicle of the Hausa-Fulani as it was seen at its creation in 1978. The strains of accomodating all of its adherents should prove to be strenuous in the coming years, particularly in a time of economic austerity with substantially reduced oil exports.

Both Awolowo's Unity Party and, to a greater degree, Azikiwe's Nigerian People's Party are seen as dropping into small, regional roles. A fourth party, the late Amino Kano's People's Redemption Party, has held onto control of only one state and will play a marginal rule for the immediate future. The other two parties only won one seat in the House of Representatives between them and that win is under court challenge.

"It is possible that Nigeria is headed for a two- or three-party system," said a western diplomat. The Unity Party and the Nigerian People's Party "may end up combining themselves into one to present an opposition to Shagari's party," he added.

Emeka Ojukwu, the former Biafran civil war military leader who returned from exile a year ago after receiving a presidential pardon, may have accounted for Shagari's strong showing in the eastern Nigeria Ibo heartland of Anambra and Imo states. Ojukwu joined Shagari's National Party and ran for a Senate seat.

Ojukwu initially lost his Senate race, but an Anambra state high court overturned the verdict in mid-September citing the erection of false polling booths and the fraudulent inflation of figures as the cause of Ojukwu's defeat.

The rioting, arson and killing of political opponents broke out during the elections in reaction to suspect election results announced by the Federal Electoral Commission. At least 100 people died in the violence and hundreds were arrested.

Two weeks after the elections ended on Sept. 3, the challenged results are just beginning to come out. Judicial panels overturned the national party victories in the gubernatorial races in Ondo and Anambra states as fraudulent and split three-to-two in upholding the National Party's win of the governor's seat in Oyo.

Many Nigerians took partisan positions on the voting fraud, blaming parties other than the one they supported.

But perhaps most revealing about the widespread electoral abuses was a statement released in mid-September by Bendel state police commissioner Potter Dabup. Among the 451 persons arrested and charged with election violations in Bendel state, Dabup said, were officials of all six parties plus officials of the Federal Electoral Commission.

Neutral diplomatic sources said all the parties were involved in the election fraud.

"So much so," added one, "that, except in some of the most blatant cases in state elections, one has the impression that all of the rigging was so widespread that it canceled out the final totals."