Participating in a debate on Parliament's budget, Mthembu said ANC MPs were deployees of their party and had to vote according to its standing decisions and principles.
He lashed out at opposition parties who have called on ANC MPs to vote according to their conscience.
ALSO READ: ANC war gets ugly
"It is very surprising that ANC members of Parliament are expected to vote against its own decisions and resolutions and party line, while all other parties are expected to vote according to their party mandates," said Mthembu.
"This is not only disingenuous and double standards...but it makes a mockery of the political party system."
He accused the DA and the EFF, South Africa's two biggest opposition parties, of extreme "hatred" of Zuma.
Mthembu indicated a vote against Zuma was a vote against the ANC.
"No party in the world has ever done so. We won't do so."