"Our morality does not allow us to desert our friends," Mandela, 80, told the president and first lady Hillary Rodham Clinton at an East Room reception with black religious leaders Tuesday night.
Without direct reference to the Lewinsky controversy, Mandela went on, "It is not our business to interfere in this matter, but we do wish to say that President Clinton is a friend of South Africa and Africa and, I believe, the friend of the great mass of black people and minorities and the disabled of the United States."
CBS News White House Correspondent Bill Plante reports that the president also received a vote of support from Bernice King, daughter of slain civil rights leader Martin Luther King Jr. "I - and, I believe, many others in this room - forgive you," she said. "I wish to say it's time for us to leave our president alone."
She then whipped Tuesday night's assembly into a chant aimed at Mr. Clinton's detractors: "Leave him alone! Leave him alone!"
After spending the night at the White House with his wife, Graca Machel, in what Mrs. Clinton called "sort of a slumber party," Mandela was being honored Wednesday with the Congressional Gold Medal in a ceremony at the Capitol Rotunda.
Machel reached for Mrs. Clinton's hand and held it.
The president called Mandela's friendship one of the greatest of the "many blessing for Hillary and me, far outweighing all the trials." Mr. Clinton joked that Mandela was only stepping down next year because "he feels like he's about 25 years old again, and he's so happily married, he can't be troubled with all these boring affairs like politics."
Earlier Tuesday, at a luncheon at the South African Embassy, Mandela said Mr. Clinton "has my full support in everything that he does. I will support my friends even if they have been deserted by the entire group."
Mandela made no reference to developments in Lesotho, where South African troops are deployed to quell weeks of political anarchy. But he did ask for U.S. investment and help in rebuilding the society and economy of his country.
He admitted the new black-run government has made mistakes since South Africa rid itself of the white supremacist apartheid system in 1994. The missteps were partly due to inexperience of the rulers and the bureaucracy, he said.
"Suddenly, without any previous training, they are called upon to run a developed country with modern ports and harbors, with modern communications...with a sophisticated banking and financial system," said Mandela.
"For you to judge us on the basis of what happens in America, in Canada, and in Europe is a grave injustice,"added Mandela. "Judge us from the background from which we come. We came from years in prison, from the bush, from exile, and from the underground."
Mandela served 27 years in apartheid prisons before his release in 1990 and subsequent election as president. As part of a series of farewells to world leaders ahead of his retirement from politics next year, Mandela leaves Washington for a swing through Canada on Wednesday.