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Defiance Campaign and Transvaal ANC Presidency: 1950–1954

Mandela took Xuma's place on the ANC national executive in March 1950, and that same year was elected national president of the ANCYL. In March, the Defend Free Speech Convention was held in Johannesburg, bringing together African, Indian and communist activists to call a May Day general strike in protest against apartheid and white minority rule. Mandela opposed the strike because it was multi-racial and not ANC-led, but a majority of black workers took part, resulting in increased police repression and the introduction of the Suppression of Communism Act, 1950, affecting the actions of all protest groups. At the ANC national conference of December 1951, he continued arguing against a racially united front, but was outvoted.

Thereafter, Mandela rejected Lembede's Africanism and embraced the idea of a multi-racial front against apartheid. Influenced by friends like Moses Kotane and by the Soviet Union 's support for wars of national liberation, his mistrust of communism broke down and he began reading literature by Karl Marx, Vladimir Lenin, and Mao Zedong, eventually embracing the Marxist philosophy of dialectical materialism. Commenting on communism, he later stated that he "found strongly drawn to the idea of a classless society which, to mind, was similar to traditional African culture where life was shared and communal." In April 1952, Mandela began work at the H.M. Basner law firm, which was owned by a communist, although his increasing commitment to work and activism meant he spent less time with his family.

In 1952, the ANC began preparation for a joint Defiance Campaign against apartheid with Indian and communist groups, founding a National Voluntary Board to recruit volunteers. The campaign was designed to follow the path of nonviolent resistance influenced by Mahatma Gandhi; some supported this for ethical reasons, but Mandela instead considered it pragmatic. At a Durban rally on 22 June, Mandela addressed an assembled crowd of 10,000 people, initiating the campaign protests for which he was arrested and briefly interned in Marshall Square prison. These events established Mandela as one of the best-known black political figures in South Africa. With further protests, the ANC's membership grew from 20,000 to 100,000 members; the government responded with mass arrests and introduced the Public Safety Act, 1953 to permit martial law. In May, authorities banned Transvaal ANC president J. B. Marks from making public appearances; unable to maintain his position, he recommended Mandela as his successor. Although Africanists opposed his candidacy, Mandela was elected to be regional president in October.

In July 1952, Mandela was arrested under the Suppression of Communism Act and stood trial as one of the 21 accused—among them Moroka, Sisulu and Yusuf Dadoo —in Johannesburg. Found guilty of "statutory communism", a term that the government used to describe most opposition to apartheid, their sentence of nine months' hard labour was suspended for two years. In December, Mandela was given a six-month ban from attending meetings or talking to more than one individual at a time, making his Transvaal ANC presidency impractical, and during this period the Defiance Campaign petered out. In September 1953, Andrew Kunene read out Mandela's "No Easy Walk to Freedom" speech at a Transvaal ANC meeting; the title was taken from a quote by Indian independence leader Jawaharlal Nehru, a seminal influence on Mandela's thought. The speech laid out a contingency plan for a scenario in which the ANC was banned. This Mandela Plan, or M-Plan, involved dividing the organisation into a cell structure with a more centralised leadership.

Mandela obtained work as an attorney for the firm Terblanche and Briggish, before moving to the liberal-run Helman and Michel, passing qualification exams to become a full-fledged attorney. In August 1953, Mandela and Tambo opened their own law firm, Mandela and Tambo, operating in downtown Johannesburg. The only African-run law firm in the country, it was popular with aggrieved black people, often dealing with cases of police brutality. Disliked by the authorities, the firm was forced to relocate to a remote location after their office permit was removed under the Group Areas Act; as a result, their clientele dwindled. As a lawyer of aristocratic heritage, Mandela was part of Johannesburg's elite black middle-class, and accorded much respect from the black community. Although a second daughter, Makaziwe Phumia, was born in May 1954, Mandela's relationship with Evelyn became strained, and she accused him of adultery. He may have had affairs with ANC member Lillian Ngoyi and secretary Ruth Mompati; various individuals close to Mandela in this period have stated that the latter bore him a child. Disgusted by her son's behaviour, Nosekeni returned to Transkei, while Evelyn embraced the Jehovah's Witnesses and rejected Mandela's preoccupation with politics.