Nigerian Vice-President Atiku Abubakar has been chosen by an opposition party, the Action Congress, as its candidate for next April's presidential election.
He was the only nominee put forward at the party's national meeting in Lagos.
He will face the ruling party's Umaru Yar'Adua if he is chosen as a joint candidate with another opposition party, the All-Nigeria Peoples' Party.
Outgoing leader Olusegun Obasanjo fell out with Mr Abubakar over a failed bid to seek an unconstitutional third term.
In September, Nigeria's anti-corruption body, the Economic and Financial Crimes Commission (EFCC), accused Mr Abubakar of fraudulently using public funds from the Petroleum Technology Development Fund (PTDF).
Mr Abubakar denies that he diverted $125m into personal business interests and says the corruption allegations are politically motivated.
In his acceptance speech, Mr Abubakar said he was pleased to accept the candidacy.
"We shall certainly overcome the present tyranny," he said, pledging to tackle issues including unemployment and corruption.
Analysts say Mr Abubakar's decision to leave the ruling party could prompt President Obasanjo to replace him, a move that could spark a constitutional crisis.
Former military ruler Muhammadu Buhari is the ANPP candidate and a committee is now expected to choose between the two northerners.
Mr Yar'Adua will be viewed as the favourite by many, but if the ANPP and AC select a joint candidate they could present a formidable challenge.
A BBC correspondent says there is little ideological difference between any of the main parties, which are mostly divided by personality and patronage.
But it seems almost certain that a northerner will win April's elections, a significant factor in a country split into a Muslim north and a largely Christian south.
Presidential candidates for some 50 registered political parties are being announced by the end of the week.
Next year's polls should become the first transfer of power from one elected leader to another since independence in 1960.
The Democratic People's Party has chosen Attahiru Bafarawa, a former northern governor, as its candidate.