ABUJA, Nigeria — When a Nigerian presidential candidate landed in the United States in January after years of being subject to a visa ban because of corruption allegations, he had a team of Western consultants and lobbyists to thank for the warm American welcome.

One of those who helped was Riva Levinson, who was mentored in the art of political consulting by Paul Manafort, the former chairman of President Trump’s presidential campaign, sentenced this month to more than seven years in prison for a host of crimes.

Ms. Levinson, who now has her own firm, KRL International, is among the many American political consultants with ties to President Trump who have become regular fixtures in African political campaigns, seizing on the region’s turn toward democracy.

For decades, countries in sub-Saharan Africa changed governments through coups that left military juntas in charge. But increasingly, in places like Nigeria, elections are supposed to determine the will of the people, though sometimes they are not fully free and fair.