Bola Tinubu, declared the winner on Wednesday in the presidential election, has boasted of making the careers of major politicians. Now he has to deliver for a divided country facing multiple emergencies.In the run-up to Nigeria’s presidential election on Saturday, the ruling party candidate’s best-known slogan was “Emi lo kan,” a phrase in the Yoruba language meaning “It’s my turn.”By Wednesday morning, his turn had finally come. Bola Tinubu, a former state governor and one of the most powerful political kingmakers in Nigeria, was declared the West African nation’s next president by election officials in the capital at around 4 a.m., after the most closely-fought contest in years.While opposition parties dismissed the election as a “sham,” alleging widespread fraud and violence and vowing to challenge the outcome in court, many Nigerians were trying to come to terms with the prospect of four years under one of the country’s most contentious figures.Widely perceived as corrupt, in poor health, and a stalwart of the old guard, Mr. Tinubu may struggle to unite a country with a huge population of young people — particularly those plugged into social media — who are increasingly trying to make themselves heard, and fighting against old ways of governing.But in Mr. Tinubu, many others see a capable pair of hands with extensive experience, who turned around Nigeria’s biggest city, Lagos, when he served as governor of Lagos State, from 1999 to 2007.A country of immense natural riches, bursting with talent — with big technology, music and film industries — Nigeria is also a nation where over 60 percent of people live in poverty, millions of children are out of school, and where kidnapping is a daily risk for Nigerians from all walks of life.A police truck drives past demonstrators accusing election officials of disenfranchising voters in downtown Abuja on Tuesday.Ben Curtis/Associated PressMr. Tinubu, a multimillionaire, says he made his money in real estate. But he has faced questions over the source of his wealth. The U.S. government took $460,000 from a bank account in his name in 1993, saying the funds were probably the proceeds of drug trafficking. He has denied any wrongdoing.He is a man of many nicknames, both reverent and irreverent. The one most often yelled at him by his supporters is “Jagaban”: meaning “big boss” or “boss of bosses,” it captures the power he wields and the deference he is often treated with as a result.But more recently, many Nigerians have taken to calling Mr. Tinubu “Balablu” — a reference to a speech in which he tried and failed to say the word “hullabaloo” — and a shorthand to imply that he is too old and sometimes not coherent enough to take on the leadership of Africa’s largest economy and one of its most complex, diverse nations. Mr. Tinubu says he is 70, but some Nigerians think he is much older.Nigerians have reason to worry about this. Their current president, Muhammadu Buhari — an octogenarian who ruled the country as a military dictator in the 1980s and returned as a democrat in 2015 — has spent much of his time in office receiving treatment in London for an illness he hasn’t disclosed.Many Nigerians did not pause to celebrate or protest Mr. Tinubu’s victory on Wednesday morning, so focused were they on surviving a cash crisis, the most recent economic shock that Mr. Buhari’s government had thrown at them.Outside an A.T.M. in Lagos — Nigeria’s biggest city — a few hours after the election result was announced, James Adah, a 38-year-old network engineer, said he had been waiting to withdraw cash for five hours. A currency redesign rolled out just before the election created a dire shortage of the new bills, leaving millions of Nigerians unable to pay for essentials, though they had money in the bank. Lines at a bank in Lagos days before the election. The government redesigned the currency, leading to widespread cash shortages just before the vote.John Wessels/Agence France-Presse — Getty ImagesThe quiet mood in Lagos reflected the overall resignation of many Nigerians, Mr. Adah said.“If people were happy you’d see jubilation,” he said. “But they’re just moving ahead amidst this perception that the election may not have been free and fair.”Mr. Tinubu won about 8.8 million votes, according to results announced in the early morning hours by the Independent National Electoral Commission, trailed by Atiku Abubakar, Nigeria’s perennial opposition candidate, with about 7 million.Not far behind, with 6.1 million, was Peter Obi, who six months ago was not seen as a serious contender in Nigeria’s traditional two-party race, but who managed to build a formidable campaign that largely grew out of a youth movement formed to protest government abuses and injustice.Mr. Obi’s and Mr. Abubakar’s opposition parties, as well as one smaller party, rejected the election results on Tuesday, calling for it to be canceled and rerun because, they said, there had been extensive vote rigging.“We won the election as Labour Party, we are going to claim our mandate,” said Yusuf Datti Baba-Ahmed, Mr. Obi’s running mate, on Wednesday. “We shall rescue Nigeria.”Questions about whether Mr. Tinubu attained the presidency fraudulently mean that he will face a legitimacy problem, according to Tunde Ajileye, a partner at SBM Intelligence, a Nigerian risk consultancy.“Any hard decisions he has to make — there are people waiting to prove that those decisions are detrimental, even if they may be right decisions,” he said. “And hard decisions need to be made about Nigeria’s economy.”A market in Abuja last month.Michele Spatari/Agence France-Presse — Getty ImagesMr. Tinubu has already promised to scrap an expensive fuel subsidy, but also has to figure out how to handle government debt and restrictions on foreign exchange, said Mr. Ajileye.Mr. Tinubu is seen by many as more capable of managing Nigeria’s oil-dependent economy than Mr. Buhari, whose tenure included two recessions.“He has a record as governor that he needs to expand nationwide,” Akeem Salau, a minibus driver, said of Mr. Tinubu on Wednesday in Lagos. “Education and infrastructure should be his priorities.”Mr. Tinubu will also face Nigeria’s multiple and mushrooming crises of security, including kidnappings, violent extremist groups like Boko Haram in the northeast and separatists in the southeast.He will have to work hard to gain the trust of the southeast, and the mostly Christian members of the Igbo ethnic group who live there, said Mucahid Durmaz, a senior West Africa analyst at the risk intelligence company Verisk Maplecroft.Most southeastern states voted overwhelmingly for Mr. Obi, who is from the region and is Christian, and against Mr. Tinubu, a southwestern Muslim who picked another Muslim as his running mate. The ticket went against Nigerian political tradition, under which one Muslim and one Christian usually run together.Peter Obi campaigning in Lagos last month.Taiwo Aina for The New York TimesIn Lagos on Wednesday afternoon, traffic flowed through the Lekki tollgate, where young people demonstrating against police brutality were gunned down by security forces in 2020. A billboard there now reads: “Vote in peace, stop electoral violence.” The Nigerian Army was accused by witnesses of having killed unarmed protesters that day, but there has been no justice for those victims, according to Amnesty International.Teniola Tayo, a policy analyst based in Abuja, said that she hoped Jagaban — the “boss of bosses” — would become accountable to Nigerians.“I hope that he will consider Nigerians his new jagabans, as he said in his acceptance speech that he is here to serve,” she said.Indeed, Mr. Tinubu took a more conciliatory tone than usual when he addressed the nation early Wednesday, reaching out to the Nigerians who didn’t vote for him, and telling the youth: “I hear you loud and clear.”A Tinubu poster in Lagos, on Wednesday.Akintunde Akinleye/EPA, via ShutterstockOladeinde Olawoyin contributed reporting. Susan Beachy contributed research. More