After a one-week delay, Africa's most populous nation goes to the polls this weekend to vote in a presidential election.

Seeking re-election is Nigerian President Muhammadu Buhari, a 76-year-old former army general who defeated Goodluck Jonathan in 2015.

Buhari is running against 71 other candidates, but his main challenger is Atiku Abubakar, a business tycoon and former vice president. Both men are Muslims who hail from the same ethnic group, but they are opposed ideologically.

Nigeria returned to democracy in 1999 after decades of military rule. When Buhari defeated Jonathan in 2015, it was the first time an incumbent had ever lost an election and there was a peaceful transition of power to the opposition leader.

Now it's Buhari who is the incumbent.

Before he was Nigeria's President, Buhari was an officer in the country's army. Here, he stands next to Saudi Sheikh Ahmed Zaki Yamani, center, and Swedish King Carl Gustaf during an OPEC summit in Stockholm, Sweden, in 1977.
Before he was Nigeria's President, Buhari was an officer in the country's army. Here, he stands next to Saudi Sheikh Ahmed Zaki Yamani, center, and Swedish King Carl Gustaf during an OPEC summit in Stockholm, Sweden, in 1977.
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As an army general, Buhari gained power in Nigeria after a coup in 1983. He was the country's head of state until 1985, when another coup forced him out.
As an army general, Buhari gained power in Nigeria after a coup in 1983. He was the country's head of state until 1985, when another coup forced him out.
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Buhari ran for president in 2003 but lost the election. He also ran unsuccessfully in 2005, 2007 and 2011 before winning in 2015.
Buhari ran for president in 2003 but lost the election. He also ran unsuccessfully in 2005, 2007 and 2011 before winning in 2015.
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Buhari salutes a crowd during a presidential rally in 2011.
Buhari salutes a crowd during a presidential rally in 2011.
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Buhari embraces his opponent, President Goodluck Jonathan, after they pledged to hold peaceful "free, fair and credible" elections in 2015.
Buhari embraces his opponent, President Goodluck Jonathan, after they pledged to hold peaceful "free, fair and credible" elections in 2015.
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Buhari casts his ballot in March 2015.
Buhari casts his ballot in March 2015.
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Buhari greets a crowd as he is sworn in to office in May 2015.
Buhari greets a crowd as he is sworn in to office in May 2015.
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Buhari is greeted by German Chancellor Angela Merkel during the G-7 summit in Germany in June 2015.
Buhari is greeted by German Chancellor Angela Merkel during the G-7 summit in Germany in June 2015.
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Buhari shakes hands with US President Barack Obama during a White House meeting in July 2015. The two leaders were expected to discuss various topics, including the fight against the Boko Haram terrorist group.
Buhari shakes hands with US President Barack Obama during a White House meeting in July 2015. The two leaders were expected to discuss various topics, including the fight against the Boko Haram terrorist group.
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Britain's Queen Elizabeth II talks to Buhari during a reception in Malta in November 2015.
Britain's Queen Elizabeth II talks to Buhari during a reception in Malta in November 2015.
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Buhari inspects a Chinese honor guard during a visit to Beijing in April 2016.
Buhari inspects a Chinese honor guard during a visit to Beijing in April 2016.
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Patricia Scotland, secretary-general of the British Commonwealth, speaks with Buhari at the start of an anti-corruption summit in London in May 2016.
Patricia Scotland, secretary-general of the British Commonwealth, speaks with Buhari at the start of an anti-corruption summit in London in May 2016.
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Buhari leaves the stage during a US-Africa Business Forum in New York in September 2016.
Buhari leaves the stage during a US-Africa Business Forum in New York in September 2016.
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Buhari meets with Chibok schoolgirls during their visit to the presidential villa in Abuja, Nigeria, in October 2016. Boko Haram militants <a href="index.php?page=&url=https%3A%2F%2Fwww.cnn.com%2F2016%2F10%2F13%2Fafrica%2Fnigeria-chibok-girls-released%2Findex.html" target="_blank">handed over 21 schoolgirls to authorities</a> after a series of negotiations, Nigeria's government said. It was the first mass release of any of the girls who were kidnapped from their school in 2014.
Buhari meets with Chibok schoolgirls during their visit to the presidential villa in Abuja, Nigeria, in October 2016. Boko Haram militants handed over 21 schoolgirls to authorities after a series of negotiations, Nigeria's government said. It was the first mass release of any of the girls who were kidnapped from their school in 2014.
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Buhari arrives at a hotel in Banjul, Gambia, to meet with West African leaders in December 2016. The meeting convened around a joint effort to persuade Gambia's longtime leader, Yahya Jammeh, <a href="index.php?page=&url=http%3A%2F%2Fedition.cnn.com%2F2016%2F12%2F02%2Fafrica%2Fgambia-yahya-jammeh-defeat%2F" target="_blank">to accept his election defeat</a> and step down from power.
Buhari arrives at a hotel in Banjul, Gambia, to meet with West African leaders in December 2016. The meeting convened around a joint effort to persuade Gambia's longtime leader, Yahya Jammeh, to accept his election defeat and step down from power.
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Buhari disembarks from a plane <a href="index.php?page=&url=http%3A%2F%2Fedition.cnn.com%2F2017%2F03%2F10%2Fafrica%2Fnigeria-president-buhari-return-medical-leave-uk%2F" target="_blank">upon his return to Nigeria</a> in March 2017. He had spent nearly two months in London receiving medical treatment for an undisclosed illness.
Buhari disembarks from a plane upon his return to Nigeria in March 2017. He had spent nearly two months in London receiving medical treatment for an undisclosed illness.
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Buhari listens to US President Donald Trump during their joint press conference at the White House in April 2018. The two leaders also met in the Oval Office to discuss a range of bilateral issues.
Buhari listens to US President Donald Trump during their joint press conference at the White House in April 2018. The two leaders also met in the Oval Office to discuss a range of bilateral issues.
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Buhari waves to a crowd of supporters during a campaign rally in Lagos, Nigeria, in February 2019.
Buhari waves to a crowd of supporters during a campaign rally in Lagos, Nigeria, in February 2019.
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