Under the Constitution, Parliament selects the president, effectively putting the decision in the hands of the A.N.C.’s top leaders.
Mr. Ramaphosa’s allies have argued that Mr. Zuma should step down as soon as possible to give Mr. Ramaphosa, as his successor, enough time to rebuild the party ahead of national elections in 2019.
The party’s decision came hours after a marathon meeting of its top leaders in the national executive committee at a hotel in Pretoria, the capital. The meeting started Monday afternoon and lasted into Tuesday’s predawn hours, as the party’s leaders tried to agree on a way to handle Mr. Zuma.
Around midnight, Mr. Ramaphosa’s motorcade was seen making its way to Mr. Zuma’s residence, where Mr. Ramaphosa directly asked for the president’s resignation, according to the local news media. After the president is said to have refused, Mr. Ramaphosa’s motorcade returned to the hotel where, in a tense meeting over the next few hours, Mr. Ramaphosa pushed members of the executive committee to formally demand that the president step down.
At an A.N.C. conference in December, Mr. Ramaphosa was elected to lead the party. But his margin of victory over Mr. Zuma’s chosen successor was slim, indicating the deep party split and presaging the difficulties he would face in pressing Mr. Zuma to step down as the nation’s leader before his term expires in mid-2019.