But he faces a stiff challenge from former vice-president Abubakar, 72, amid fears about widening insecurity, claims of creeping authoritarianism and economic incompetence. Analysts were split over who would win Saturday's ballot, which is the sixth in the 20 years since Nigeria returned to democracy after decades of miliary rule.
"It's likely to be very, very tight," said Nnamdi Obasi, senior Nigeria researcher at the International Crisis Group think-tank. "Initially the president and the ruling party were way ahead in the race but in recent times the opposition has been seen to make a more vigorous and more robust campaign.
"We can't say exactly how it will go," he told BBC World Service radio. A record number of 73 candidates are on the ballot but Buhari, from the All Progressives Congress (APC), and Abubakar, of the Peoples Democratic Party (PDP), are considered the main contenders.
Over 84 million people are registered to vote - up 18 percent on 2015 and another record, which has been taken as a sign of Nigeria's developing democracy. Also up for grabs are 360 seats in the lower House of Representatives and 109 in the Senate.
Nearly 120,000 polling units are set to open at 0700 GMT and close at 1600 GMT. No date has been given for the results, but an announcement is expected from early next week. Electors face a choice between two elderly candidates who have both been part of the political elite for decades and do not mirror the country's increasingly young demographic.
Just over half the registered voters are aged 18-35. Chief among the criticisms against Buhari is security, with signs of a resurgence of Boko Haram in Nigeria's remote northeast and new conflicts elsewhere.
His anti-corruption campaign has been described as one-sided, unduly targeting political opponents. Economic growth picked up last year after a recession in 2016 but remains sluggish. The cost of living is high in a country where most of the 190 million people live in poverty despite billions earned from oil.
Abubakar, 72, bills himself as a dynamic, modern, pro-business leader. But the former vice-president faces allegations about links to corruption. Buhari said in a televised address Thursday that reelection would give him the chance to fulfil his initial promises and complete vital infrastructure projects.
"There is no best candidate among them," said Aliyu Jibrilla, a 70-year-old retired teacher in the Adamawa state capital, Yola, adding: "Intellectually. they're not up to it". "It's about time these old people go," added Modibbo Sadiq, a 23-year-old university graduate. In Buhari's home town of Daura, in Katsina state, Abdulaziz Abdullahi agreed. "Ideally, the young generation should be in charge of country," said the 25-year-old, who sells glasses.