When Nelson Mandela ascends the podium this morning to speak to a joint meeting of the House and Senate, he will be the third former political prisoner to stand before Congress within the last eight months, following recent appearances by Polish Solidarity Chairman Lech Walesa and Czechoslovakian President Vaclav Havel.

Mandela, however, brings not only the image of a freedom fighter but a political agenda that makes him among the most controversial foreign leaders ever to speak to a joint meeting, according to a congressional historian.

"Generally, Congress has tried to stick to safe people," said Mary Lee Kerr, a research consultant for the Institute for Southern Studies in Durham, N.C. "The Reagan era especially was a pretty safe era that invited safe people. But as the messages of people like Mandela become more acceptable in the world . . . these people are becoming more acceptable in the eyes of Congress."

Mandela is expected to receive a hero's welcome in Congress, which voted for economic sanctions against South Africa in 1986. Though he created some tension last week when he reiterated his support of Yasser Arafat at a time when the U.S. government was calling off its dialogue with the PLO leader, spokesmen for the House and Senate leadership say that there has been no strong opposition to Mandela's appearance.

"Why should we expect Mandela to articulate U.S. foreign policy?" asked Edwin Dorn, deputy director of research at the Joint Center for Political and Economic Studies. "Some of his comments will produce detractors, nevertheless I think there is a widespread willingness to give this genuinely heroic figure the benefit of the doubt . . . given the nature of his struggle."

Historian Kerr believes that the invitation to Mandela, the first South African to be received by Congress since former Prime Minister Jan Christian Smuts in 1930, may signal a return to a practice in much earlier Congresses to seek out controversial figures.

An 1852 visitor, Louis Kossuth, was an exiled governor of Hungary who had tried to topple Hapsburg rule in his country. Charles Stuart Parnell, an Irish politician who led the Home Rule movement in the 1880s, spoke to Congress about reforming the land-tenure system in Ireland, a belief for which the British government later imprisoned him.

"It's truly the most prestigious podium to speak to the world," said Fred Schwengel, president of the U.S. Capitol Historical Society, a nonprofit educational organization operating under a government charter to promote the history of Congress. Schwengel, a former Republican House member from Iowa, oversaw the society's publication last year of a compilation of the nearly 140 speeches given by foreign dignitaries to Congress.

The historical legacy of the addresses first attracted Schwengel's attention when he discovered that the portrait of the Marquis de Lafayette hanging in the House chambers was placed there in memory of his appearance before Congress in 1824. Lafayette's speech inaugurated the tradition of Congress providing a platform for visiting foreign dignitaries to express their political views, a practice unlike any in the world.

These foreign speakers range from the illustrious -- Winston Churchill, Charles de Gaulle and Jawaharlal Nehru -- to those who became notorious -- Anastasio Somoza Garcia, the patriarch of the Nicaraguan dynasty, and Ferdinand Marcos of the Philippines, who has the distinction of delivering the longest speech.

There are also those whom history has largely forgot, such as Boris Bakhmeteff, a representative of the provisional Russian government who spoke in June 1917 of a Russia that had, since the overthrow of Czar Nicholas II, "lived through events of world-wide importance," and that was, he said, "joining the ranks of democracy."

Because joint meetings are traditionally convened by unanimous consent, through the years an informal arrangement has developed between the House and Senate leadership and the White House to secure this consensus before issuing an invitation.

As political as this process is, it has remained free of controversy except on rare occasions, such as the 1987 summit visit by Soviet President Mikhail Gorbachev. A plan to invite him to speak to a joint meeting was dropped after some members of Congress voiced opposition to granting the head of a communist government the symbolic center stage of Congress.