Peter Obi and Rabiu Kwankwaso, who came third and fourth respectively in the last elections, switch parties.
Related stories
Dozens of Australians believed to be among hundreds of foreign tourists missing in Nepal flash floods
Dolly Parton's Life, in Pictures
What we know about the deadly flooding on the Nepal-Tibet border
Teen receives life sentence after pleading guilty to murdering 10-year-old cousin
FBI probe into cyberattacks on US water systems expands after tech supplier is targeted
Trump has still not picked a Karoline Leavitt replacement days before she leaves