Mr. Babangida too speaks of the need for a new type of leadership, saying his time out of office has made him better prepared to return to power.

"I believe strongly that in the midst of the present signals there is hope for a foundational renewal," Babangida said. "I am certain that Nigerians have the capability to rise to the demands of rapid development and positive change under a competent, visionary, bold, fair, firm, honest, purpose-driven leadership."

President Jonathan says he intends to run a positive campaign to unite the country behind improving education, electricity, and water.

"I have come to preach love not hate, I have come to break you away from divisive tendencies of the past, which have stopped our drive to true nationhood," Mr. Jonathan said. "I have no enemies to fight, you are all my friends and we share a common destiny."

President Jonathan's election bid upsets an informal ruling-party deal that rotates power between north and south every eight years. Under that plan, the next Nigerian president should be from the north to finish out what would have been President Yar'Adua's second term instead of continuing on with President Jonathan, who is from the south.

Candidates face other challenges

In addition to Mr. Babangida, Mr. Jonathan is facing ruling-party challenges from former vice president Atiku Abubakar and former security advisor Aliyu Gusau.