Until last month, Nigerian presidential candidate Atiku Abubakar had a problem. He was persona non grata in the U.S. after cropping up in connection with several corruption investigations.

Then the cloud lifted. Years after he'd last been seen in the U.S., Abubakar surfaced in Washington in January. He held court at the Trump International Hotel. He met with members of the U.S. Chamber of Commerce and Congress. Those meetings were trumpeted to his followers back home on Facebook and Twitter. The public tour has helped silence opponents who said Abubakar couldn't effectively lead one of Africa's biggest economies if he wasn't even welcome in the U.S.

Abubakar had been blocked from entering the U.S. under a State Department edict applying to officials linked to foreign corruption, two former U.S. officials said. One of them said the Nigerian had been seeking a waiver to enter the country for years and expressed surprise when told that the effort was ultimately successful.