A senior politician in South Africa’s historically white main opposition Democratic Alliance (DA), Herman Mashaba has resigned his post and quit the party over how it handles issues relating to race.

Mashaba’s election as Mayor of Johannesburg from the DA was widely seen as a sign that the opposition party could potentially threaten the ruling African National Congress (ANC) at national level.

As the first non-ANC Mayor of South Africa’s biggest city since the end of white minority rule in 1994, Mr. Mashaba’s election victory in 2016 signaled a major success for the DA.

“I cannot reconcile myself with people who believe that race is not important in their discussion of inequalities,” Mashaba said at a press conference on Monday.

He said his decision to resign was sparked by the re-admission of Helen Zille, a white politician who provoked widespread anger in 2017 when she praised aspects of colonialism to the party’s high ranks.

“The election of Zille as Chair of Federal Council is victory for people who are opposed to my belief systems,” Mashaba added.

Black leaders within the DA believe that the liberal, multi-racial party was gradually going back to its roots of being an all-white organization.

Mashaba also said the South African government’s pro-poor agenda has been undermined, criticized and rendered nearly impossible by the DA. 

A self-made businessman, Mashaba was seen as a key ally of the party’s first black leader, Mmusi Msimane in his efforts to increase support among black voters.

However, the DA’s share of the vote dropped in the May general election after conservative whites abandoned the party and Zille’s return to a senior post in the party is seen as an attempt to regain their support.

Mr. Mashaba said he would step down next month, raising questions about whether the DA would be able to retain the mayoral post in Johannesburg.