It was unfortunate that Zille justified the merger with the ID by "peddling misleading lies about corruption thriving under the ANC's governance and about the Constitution being under threat", he said.
"These are tired scarecrow tactics that could not sustain Cope and would not hold (the) DA-ID marriage together."
The DA and ID have reached an understanding which could see the two parties merge in 2014.
Announcing the step on Saturday, Zille said: "We've realised that the ruling party has become increasingly corrupt and is abusing the Constitution."
Mthembu said the ANC had made it fashionable to fight corruption and was doing so successfully.
"We have a big arsenal of instruments to fight corruption within the State, and they are performing their functions very effectively without fear or favour."
He urged ID any members who did not support the parties' co-operation pact to join the ANC.