NAIROBI, Kenya — What appeared to be a historic step forward for democracy in Africa was thrown into turmoil on Tuesday when an opposition leader said he would not participate in a redo of Kenya’s presidential election.

“All indications are that the election scheduled for Oct. 26 will be worse than the previous one,” the opposition leader, Raila Odinga, said at a news conference, because the electoral commission did not intend “to undertake any changes to its operations and personnel” to ensure a free and credible election.

The announcement came a month and a half after the Kenyan Supreme Court made history by nullifying the election of the incumbent, President Uhuru Kenyatta, and ordered a new one, in response to complaints of voting irregularities.

“The Supreme Court decision in September was unpredictable, and it is still unpredictable what it will lead to this time,” said Justin Willis, professor of modern African history at Durham University in Britain.