The powerful head of the Nigerian senate, Bukola Saraki, on Thursday announced he would run in next February's presidential elections against incumbent Muhammadu Buhari.
The move, by the country's third-highest politician, ratchets up the challenge facing Buhari and increases the electoral uncertainties in Africa's most populous country.
"I hereby announce my intention to run for the office of President of the Federal Republic of Nigeria in the coming general elections in 2019 on the platform of the People's Democratic Party," Saraki said at a meeting with young people in Abuja.
"I do so with the firm conviction that I have what it takes to secure inclusive growth for Nigeria and Nigerians." Saraki, 55, was born into one of Nigeria's wealthiest and most politically prominent families.
Ranked in Nigeria's political hierarchy behind Buhari and Vice President Yemi Osinbajo, Saraki has long been seen as having his sights on the top job. In recent months, he became linked to a string of defections that appeared to be aimed at weakening Buhari.
On July 31, citing irreconcilable differences, he quit Buhari's All Progressives Congress (APC) and returned to his old party, the PDP.
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During his time as Senate president, Saraki has had a running battle with the government. He ran into legal problems that he said was because he had been marked out as a critic and rival by the presidency.
Buhari, 75, a former military dictator in the 1980s, was elected in 2015. He has announced his intention to seek a second term. He has the support of the APC's senior ranks, although a primary is required.
But he is under intense pressure to stand down on account of age and failing health and faces criticism of his handling of the Boko Haram jihadist insurgency in northeastern Nigeria.
Last year, he spent several months in a London hospital receiving treatment for an undisclosed ailment.
On Wednesday, another heavyweight candidate said he would be a candidate in the PDP primaries -- Rabiu Kwankwaso, a former aide to Buhari and ex-governor of Kano province, who defected to the PDP in July.