Mr. Atiku Abubakar touts himself as the most experienced person in the election, having been the vice president between 1999 and 2007 – and it was this time that there was some controversy.
His boss former president Olusegun Obasanjo accused him of embezzling $145 million from the Petroleum Technology Development Fund (PTDF) in 2003.
Mr. Obasanjo, who is quick to speak out against other politicians, included a chapter in his book My Watch, published in 2014, on the alleged wrongdoing of his former deputy.
Mr. Abubakar denied the allegation, saying $145 million had been invested in commercial banks to accumulate interest to be reinvested in the PTDF project.
The 76-year-old said he first made money from farming and owning houses in his home state of Adamawa. A former customs officer, he said that he recognized early in life that he had “a good nose for money”.
It was in the 1980s that he established an oil-servicing firm that catapulted him into the world of the wealthy.
Opponents have accused him of flouting a law that prohibits civil servants from engaging in private business other than farming.
Mr. Abubakar’s spokesperson described his venture as a small business that many public servants engage in, such as using their car as a taxi or setting up a shop in front of their house to support their families.
“He only invested his income to earn interest. He was not doing any other work in form of private work like you are inferring,” Paul Ibe told the newsmen.
In 2010, a US Senate committee report alleged that between 2000 and 2008, Mr. Abubakar, through one of his four wives, transferred more than $40m in “suspect funds” into the US from offshore shell companies.
The report claimed that at least $1.7m of this came from bribes paid by German technology company Siemens, which pleaded guilty to bribery charges in 2008 and agreed to pay a $1.6bn fine.
He was also a major figure in the corruption trial of former US Congressman William Jefferson, who in the Senate report described Mr. Abubakar as “really corrupt” and said he needed money to bribe him to approve a US company’s business deals in Nigeria.
Mr. Jefferson was convicted in 2009 and sentenced to 13 years in prison, which was subsequently reduced.
Mr. Abubakar has repeatedly denied any wrongdoing and neither he nor his now-divorced wife face criminal charges in the US.
“Atiku Abubakar is not on trial for corruption or any other misconduct either in Nigeria or any foreign land,” Mr. Ibe said.
“He can only be unfit for public office when he is indicted by a court of law. That is not the situation.”