President Obama on Thursday kick-started his trek through sub-Saharan Africa with a call for democracy in a region long defined by human rights violations and corrupt government regimes, looking for a transformational moment that many have expected from the nation's first African-American president.

But Obama's mission isn't so simple, particularly as the region remains fixated on the deteriorating health of former South African President Nelson Mandela and as culture clashes came to a head at the outset of his trip.

"Obviously for an African American -- and an African American president -- to be able to visit this site I think gives me even greater motivation in terms of the defense of human rights around the world," Obama said, peering out from the "Door of No Return," a former slave-trading post on Senegal's Goree Island.

For all the talk of progress in Senegal, however, the wide divide between the two countries was on full display Thursday.

As Obama praised the Supreme Court's decision to overturn the Defense of Marriage Act, he did so in a country that still outlaws homosexuality. When pressed by an American reporter about the harsh laws, Senegalese President Macky Sall said his country was not homophobic but that it was not ready to decriminalize gay acts.

And the destinations missing from Obama's itinerary say as much about his endgame in Africa as those he will visit.

For example, Obama won't visit Kenya -- his father's homeland -- because the leader there has been charged with war crimes. Nor will Obama go to Nigeria, which is under martial law.

"The countries to visit were carefully chosen; the three largest by population -- Nigeria, Ethiopia and the Democratic Republic of the Congo -- all have serious governance and security issues, and none is a model of the rule of law," said John Campbell, a former U.S. ambassador to Nigeria and senior fellow at the Council on Foreign Relations. "Kenya is an important ally, as well as the country of the president's father. But its president and vice president are under indictment by the International Criminal Court."

Obama was happy Thursday to keep the focus on Senegal, a country that has made more democratic strides than many of its African neighbors.

As he hopscotched between Senegalese landmarks, including the presidential palace and the nation's highest court, crowds long clamoring for a presidential visit waved signs that read, "Welcome home."

The entire trip is being cast in the shadow of Mandela, the icon of the anti-apartheid movement, who has been in the hospital for 20 days with a lung infection.

Obama has long called Mandela, South Africa's first black president, a personal hero. But the two haven't met face to face aside from a brief get together in Washington.

"My first act of political activism was when I was at Occidental College," Obama said Thursday, reflecting on how Mandela influenced him. "I got involved in the anti-apartheid movement, back in 1979-80 because I was inspired by what was taking place in South Africa."

Mandela remains in stable but critical condition, his family said. The White House said Mandela's family would decide whether he is healthy enough to visit with Obama over the weekend. Obama is scheduled visit South Africa on Friday, where he will focus primarily on food security and efforts to increase trade between the two nations.