"Everyone seems to be missing Helen's role in obliterating DA's chances with black voters. Her comments about colonialism obliterated the trust we had worked hard to build," said a DA leader who preferred anonymity.
Zille came under fire last year for suggesting that colonialism did not only have a negative impact.
"Do you genuinely believe the legacy of colonialism was ONLY negative?" she asked in a tweet.
A DA leader said: "People don't forget and the trust, which is our main blockage to gain support of black voters, was destroyed by her".
In her reply, Zille said: "Just go and look at the results, disaggregate the voting patterns, do the proper analysis and you won't take this kind of nonsense seriously. Analyse the statistics. That is the only way to do it".
The DA suffered a 1.46% decline in its support during the national and provincial elections, which saw the party losing five seats in National Assembly. This was the first time since 2004 that the party's support has dropped.
"We made it clear that he's not going anywhere," said FedEx member and provincial leader in KwaZulu-Natal Zwakele Mncwango.
"We never pinned any decision on Mmusi because no one takes a decision alone. We are a collective and we take a collective responsibility," Mncwango said.
Another FedEx member, Yusuf Cassim, said they were concerned about the decline in numbers and that they were aware that some of their supporters either stayed at home during the polls, while some voted for the Freedom Front Plus, but no one blamed Maimane for this.
"We are concerned [about the decline] and we will go back to get those votes, but no one has said that Mmusi must step down. Actually, when Mmusi spoke everyone applauded," said Cassim.
"What Mmusi was saying was about the message of building one South Africa and that we are not going to abandon it. We're fighting for one SA and that mission is historic."