JOHANNESBURG -- Nelson Mandela, the legendary symbol of black defiance of South Africa's apartheid system, arrives today in the United States on a mission aimed at winning recognition of his African National Congress as the probable future ruler of this nation.

Mandela will become the first ANC leader ever to visit the White House, which until now has held his organization at arm's length because of its Soviet backing and socialist orientation. A shift in U.S. attitude toward the ANC only began in January 1987, when George P. Shultz, then secretary of state, met ANC President Oliver Tambo at the State Department.

But Tambo was not invited to the White House because of conservative pressure on President Ronald Reagan. Tambo is now recovering from a serious stroke in Sweden, effectively leaving Mandela, whose formal title is deputy president, as the ANC's leader.

During his 12-day whirlwind tour of the United States, which begins in New York, Mandela is expected to argue for continuation of U.S. sanctions to help the ANC keep pressure on South African President Frederik W. de Klerk for authentic black majority rule. Mandela recently praised the 1986 Comprehensive Anti-Apartheid Act as the only sanctions legislation anywhere in the world.

Mandela's grueling six-week tour of Western Europe and North America is aimed at preventing the easing of sanctions in response to de Klerk's initial and promised future reforms. De Klerk has just broken South Africa's 30 years of diplomatic isolation by meeting with nine European leaders, seeking to persuade them that his reforms are irreversible. The European Community is now heatedly debating whether to begin lifting some of its economic or other sanctions.

With its armed struggle likely to be "suspended" shortly, the ANC regards foreign sanctions as the best form of pressure available to it to ensure that negotiations lead to real black majority rule.

The ANC will also use Mandela's enormous prestige during his visit to eight U.S. cities to raise millions of dollars to help the newly legalized movement build a party infrastructure across South Africa and resettle more than 20,000 returning exiles.

Congress recently appropriated $10 million to support the emerging democratic process here, and the Democratic Party wants this to be channeled through the National Endowment for Democracy partly to help the ANC and other black nationalist groups here get organized.

Americans are likely to find Mandela an engaging personage. After languishing nearly 28 years in prison, he has emerged at age 71 with instant commanding authority over his own party, unusual energy and stamina and a single-minded sense of historic purpose to lead his people to the promised land of black rule.

Even before leaving prison, Mandela had made clear to white leaders of the ruling National Party in a series of private talks that he was the black leader with whom they could negotiate.

Mandela combines a certain aloofness and British reserve with an inner warmth and personal charm that often captivate those in his presence. He has an old world courtliness and dignity, while his frankness in the face of even embarrassing journalistic interrogation is remarkable among black African leaders.

He has not shied away from answering questions about allegations of human-rights abuses within the movement leveled by dissident ANC members or about charges of serious misconduct by his wife, Winnie, that were raised in the recent murder trial of a young Soweto township activist.

In four months of nearly constant rallies, news conferences, interviews and public appearances since he was set free Feb. 11, Mandela has never shown the slightest irritation or lost his sense of humor.

His tall, ramrod-straight figure has already become familiar in South African homes now that state-run television has begun featuring him after nearly three decades during which the law prohibited any picture of him on the screen or in print.

Since his release, Mandela has almost singlehandedly established the ANC as the dominant black political party in South Africa, holding the biggest rallies ever seen here as he has moved across this land. He has probably addressed at least a million people, drawing crowds of more than 100,000 awed admirers again and again.

In a striking indication of the relative strengths of South Africa's black political movements, a crowd of about 35,000 packed Soweto's Jabulani Stadium Saturday for a rally commemorating the 1976 Soweto uprising, while only about 1,000 attended a rally of the rival Pan-Africanist Congress in the much larger Orlando Stadium.

Many blacks call Mandela "our father" and regard him as a political savior from the oppression of apartheid and white rule.

While his personal popularity is an enormous asset, Mandela's health is becoming a serious concern as the ANC presses to make use of his enormous prestige to establish itself at home and abroad as the future ruling party of South Africa. His nearly nonstop schedule of rallies, meetings and trips since his release seems to be starting to take a toll on his stamina. His schedule in Europe had to be rearranged several times to allow more rest.

After suffering tuberculosis and undergoing a prostate gland operation while in prison, Mandela has just had a nonmalignant cyst removed from his gall bladder. He began this trip two days after leaving the hospital.

{Washington Post correspondent William Claiborne reported from Toronto that Mandela was 90 minutes late for an appearance at a high school Tuesday because, his aides said, he needed rest. His only other appearance in Toronto was before a group of local anti-apartheid activists at his hotel.

{Jabu Jube, the local ANC information officer, warned reporters in Canada Monday that some scheduled events might have to be canceled, adding, "We are taking fatigue into account."}

If Mandela were to become seriously ill, his hospitalization on top of Tambo's would be a disaster for the ANC, which is already facing serious internal problems.

Key decisions need to be processed through its bureaucratic National Executive Committee, whose 35 members are scattered around the globe, with principal leaders in Stockholm, the Zambian capital of Lusaka and Johannesburg. This separation has made the ANC extremely slow in exploiting the new political freedom here. As the exiled ANC foreign affairs chief, Thabo Mbeki, remarked during a visit here last month, "We coming from Lusaka have been shouting to our comrades in Johannesburg to say, 'Why are you moving so slowly?' "

The ANC has launched a drive to enroll 2 million members, but it is still having trouble just answering phone calls and questions at its head office here. It has been hampered partly by the continued exile of most of its top officials and partly, too, by the problems of meshing a welter of semi-autonomous pro-ANC groups and their youthful leaders into party organs dominated by an older generation.

"Until it is thoroughly professionalized and systematically organized, the ANC's structures will be unable to translate mass support into organized power," Mark Swilling and Johannes Rantete of the Witwatersrand University Center for Policy Studies wrote recently.

Mandela has found himself at the center of controversy in the white community because of his statements defending the old ANC policy of seeking to nationalize "the commanding heights of the economy," a code word for South Africa's powerful mining companies and banks.

Government and big business here are now in the midst of a campaign to "privatize" many major state-owned industries and services, such as steel and transportation. The ANC's call for nationalization has been derided as showing naive disregard for the current lessons of Eastern Europe, where socialism seems headed for the dustbin of history.

But the ANC, noting that 87 percent of South Africa's land is owned by whites and that 95 percent of industrial enterprises are in white hands, is demanding a serious redistribution of the country's resources. It is also talking about breaking up private monopolies in a country where six giant companies account for 87 percent of the capitalized value of the Johannesburg stock exchange.

Mandela has effectively used the nationalization threat to force the white government and business to begin facing up to the unresolved economic and social problems in which the black population is mired. He has provoked a major public debate even while the ANC has quietly undertaken several studies of the economy in light of socialism's failure in Eastern Europe.

"The ANC has no blueprint that decrees that these or other assets {land and public corporations} will be nationalized," Mandela told a business conference recently. But, he added, the idea of nationalization "should not be ruled out of the court of discussion."

Equally controversial has been Mandela's defense of the ANC's close alliance with the South African Communist Party, which whites here tend to blame for the ANC's advocacy of socialism. Africa Confidential, a British publication, recently estimated that 27 of the ANC's National Executive Committee's 35 members belong to the Communist Party.

ANC officials will neither confirm nor deny the report, saying they don't keep such statistics and demand only that members adhere to the ANC's policies and objectives. Nobody has suggested that Mandela was ever a party member, but his wife sometimes wears a party badge.

Western diplomats seem unconcerned about the ANC-Communist alliance. "What does it mean to be a Communist today anyway?" one remarked recently.