EXPLAINER-The race for Nigeria's presidency in 2019

ABUJA, Feb 21 (Reuters) - Nigeria's elections were postponed by a week to Feb. 23 just five hours before polling units were set to open last Saturday, causing shock and disappointment among many voters.

President Muhammadu Buhari and his rival, former vice president Atiku Abubakar, urged voters to stay calm.

Both leading candidates' parties accused each other of being behind the delay and colluding with figures within the electoral commission. Neither party has provided evidence for this.

The electoral commission said the postponement was due solely to logistical factors and denied political pressure had played any part in the decision.

This is the third straight election to be postponed. Presidential elections in 2011 and 2015 were also delayed over logistics and security issues.

Nigeria will on Saturday choose between Buhari and Atiku, both in their 70s. Between them, the two candidates have run for president nine times. Almost 70 other candidates are running, though their chances are slim.

WHAT'S AT STAKE?

Buhari's 2015 victory with his All Progressives Congress (APC) party was built on three promises: to rid Nigeria of corruption, fix the economy and defeat threats to security.

Results have been mixed. The military's efforts to fight the Boko Haram Islamist insurgency, now in its tenth year, are crumbling.

The economy entered and climbed out of recession under Buhari, yet Nigerians are getting poorer. Opponents say his government is failing to tackle corruption, targeting only his enemies and ignoring allegations against his allies.

Many have accused the former military dictator's administration of flouting the rule of law. The army has been accused of human rights abuses, including the massacre of protesters. Court orders have been ignored by the executive branch and security services have arrested journalists.

After he spent five months in Britain in 2017 receiving treatment for an undisclosed ailment, opposition groups said Buhari was unfit for office and his administration did little.

If Buhari wins, his opponents say Nigeria faces another four years of political torpor and disregard for rights.

The president's campaign has focused on rail and road development that he says would reap long-term benefits. He has also vowed to expand a vocational skills programme, improve access to credit for entrepreneurs, and fight corruption.

The alternative is Atiku, of the opposition People's Democratic Party (PDP), who has faced corruption allegations since serving as vice president from 1999-2007. He has denied wrongdoing.

Atiku, who has ventures ranging from a port logistics firm to a private university, has said he would introduce business-friendly policies aimed at doubling the size of the economy by 2025.

His proposals include privatising parts of the state oil company and creating a $25 billion fund to support private sector infrastructure investment.

Atiku's opponents say he would exploit those policies to enrich himself and those around him.

FAULT LINES AND FRACTURES

Nigeria is deeply divided. One of the most fundamental rifts is between the mainly Muslim north and the largely Christian south, and the population is fairly evenly split between the religions. Africa's most populous country has more than 200 ethnic groups, with the three largest the Hausa in the north, the Yoruba in the southwest and the Igbo in the southeast.

That has led to an unofficial power-sharing agreement. The presidency is to alternate between north and south after every two four-year terms. Buhari, a northern Muslim, has held the post since 2015. His predecessor, the PDP's Goodluck Jonathan, is a southern Christian. In keeping with the accord, the PDP selected Atiku, a northerner, as its candidate for 2019.

With two northern Muslims leading the pack, there may be less election violence around ethnicity, religion and region. However, the south has favoured the PDP in the past, while the north is Buhari's stronghold.

But in the diverse hinterland states known as the Middle Belt, the picture is less clear. Long-running violence over diminishing arable land between farmers and nomadic herders has exploded, with a death toll of more than 3,600 since 2016, according to Amnesty International.

This could turn the Middle Belt, much of which voted for Buhari in 2015, into swing states.

GENERATION GAP

With a booming young population, Nigeria's median age is just 18. Many youth see Nigeria's ageing leaders as out of touch. Buhari, 76, is the oldest person to lead Nigeria since the return to civilian government in 1999. That sparked "Not Too Young to Run" campaigns to allow younger people to seek office. Despite that, the PDP chose 72-year-old Atiku as their candidate.

Nigeria's former military leaders also retain a strong influence over politics nearly two decades after the return of civilian rule. Buhari, then a general, took power in a military coup in 1983 and ruled until 1985.

THE PARTIES

The two main parties do not have clear ideological differences. Competition for control of oil revenues, patronage and rivalries between ethnic groups have played a much bigger role in elections than ideology.

However, Buhari's 2019 campaign has prioritised poverty alleviation and social schemes, while Atiku has stressed the need for a better business environment. Both candidates vow to improve the country's decrepit infrastructure.

The APC is the latest incarnation of the various vehicles Buhari used to run in 2003, 2007 and 2011. His eventual victory in 2015 came after assembling a broad coalition across the north and southwest.

The PDP was the inheritor of decades of military rule in 1999, and held power until Jonathan's defeat in 2015. The party has traditionally appealed to Nigeria's business community.

Atiku, a long-time PDP member, joined the APC and became a key ally and funder of Buhari during the 2015 campaign. Atiku switched sides in late 2017.

ELECTION INTERFERENCE

Concerns about election rigging are high. Buhari has said anyone trying to tamper with the vote, "will do it at the expense of his own life". The PDP said Buhari was calling for "jungle justice".

In January, Buhari triggered a constitutional row when he suspended the chief judge, who has a crucial say in resolving election result disputes, over his alleged violation of wealth declaration rules.

Nigerian social media has become just as fraught. Rumours are flying thick and fast online despite a government campaign to tackle "fake news". Responding to the problem, Facebook said it would roll out some of its political advertising rules and tools for curbing election interference to Nigeria.

TURNOUT AND RUNOFFS

Voter turnout in the 2015 election was 29.4 million, or 44 percent of the 67.4 million registered voters, according to Independent National Electoral Commission data. For 2019, the number of registered voters has risen to 84 million, just over half of them aged 18-35.

The candidate with the most votes is declared winner as long as they have at least one-quarter of the vote in two-thirds of Nigeria's 36 states and the capital. Otherwise there is a run-off. (Writing by Paul Carsten Editing by Alexis Akwagyiram and Giles Elgood)