(31.März 2015) Since the end of military rule in 1999, Nigeria has been governed by a single, dominant party — Mr. Jonathan’s Peoples Democratic Party. But on Tuesday, Mr. Buhari’s campaign said that the president had called the challenger at 5:15 p.m. to acknowledge defeat — an act many Nigerians considered a big step forward after previous elections marred by fraud and incumbents clinging to office.