Let's take a closer look at his history with the united states. He had a deep impact on our politics before setting foot on our soil. Here's jonathan karl with more. On how mandela prodded, consoles,... See More
Let's take a closer look at his history with the united states. He had a deep impact on our politics before setting foot on our soil. Here's jonathan karl with more. On how mandela prodded, consoles, scolded, and inspired american presidents. Reporter: Nelson mandela loomed large in america long before he was freed from prison. ♪ It was 25 years ♪ ♪ that take that man away ♪ Reporter: Inspiring a mass movement against racism and intolerance. Apartheid, no. Freedom, yes. Reporter: His relationship with u.S. Presidents has been far more complicated. When he was locked up in 1962, the u.S. Government was silent. In 1966, bobby kennedy went to south africa and took a stand against racism. Giving the greatest speech he ever delivered. Each time a man stands up for an ideal, he sends forth a tiny ripple of hope. Reporter: From lbj to nixon to jimmy carter, south africa's apartheid government was actually a u.S. Ally in the cold war. As the anti-apartheid movement grew, a young college student named barack obama was inspired by mandela to give his first political speech. The man in the white house then said no to sanctions. Against south africa, insisting they wouldn't work. Congress defied ronald reagan and imposed them any way. Reagan took his own stand against apartheid by appointing the first black man as ambassador to africa. Four years later, mandela was free. His first visit to america. Warmly welcomed at the white house. Mr. Mandela, a man who embodies the hopes of millions. Reporter: It was bill clinton with whom he would develop the closest bond. Mandela, now president of south africa, visited the white house during the darkest days of the clinton presidency. He gave his friend a boost. Our morality does not allow us to desert our friends. Reporter: This friendship clinton treasures to this day. We just hit it off. I just adored him. He was always, you know, he was a true friend. Reporter: Mandela, an ex-president, met with george w. Bush in 2005. But there was no love lost there. Mandela was one of bush's harshest critics when it came to iraq. When we talked to bush about the ailing mandela earlier this year, there were no hard feelings. He promoted freedom. He was a really great leader. He was smart and capable. And made his mark. Reporter: Obama only met mandela once. Ever so briefly as a junior senator. But his connection may be the most profound. It was mandela, he says, who awakened him to the wider world. Inspiring him to political activism. He gave me a sense of what human beings can do when guided by their hopes, not by their fears. Reporter: In other words, there night not be a president obama if not for nelson mandela. For "this week." Jonathan karl, abc news, washington.
This transcript has been automatically generated and may not be 100% accurate.