The necessity for Robert Mugabe to appoint a vice-president is constitutional dealbreaker for party politburo.
Zanu-PF is blocking a draft constitution that would dilute President Robert Mugabe's powers and force him to name a successor, stalling a key step towards reform in Zimbabwe. And it is believed that Movement for Democratic Change (MDC) leader Morgan Tsvangirai may also back the scrapping of the clause, even though his party is in favour of the reform.
Ahead of the Southern African Development Community (SADC) summit in Maputo this week, the impasse has increased pressure on President Jacob Zuma to act. Zuma's visit to Harare this week was, according to his spokesperson Mac Maharaj, meant to ensure Zimbabwe remained "on the route to a fair, intimidation-free election". Zuma is SADC's mediator in the Zimbabwe impasse.
However, positions appear to be hardening within Mugabe's inner circle, which believes constitutional reform is being used to nudge Mugabe out of power. Zanu-PF's representatives to an inter-party panel drawing up a new constitution had agreed to and signed the draft, but now the party said it must be overhauled. Mugabe's top council, the Zanu-PF politburo, has held a series of marathon meetings on the draft, many of them lasting through the night.
A key dispute area is a clause that would compel a presidential candidate to name a running mate. The president would be asked to name his "first vice-president", who would automatically succeed him should he leave office before the end of his term.
If the draft passes and Mugabe names his first vice-president, he will in effect be anointing a successor. Mugabe has avoided doing so for years, instead ensuring his survival by playing rivals off against each other while convincing supporters that only he can hold the party together. He has previously said he had considered retirement, but naming a successor would tear his faction-riddled party apart.
Although both factions of the MDC have backed the draft, Tsvangirai, facing his own factional battles, may secretly be in favour of scrapping the clause. Reports have suggested Tsvangirai might name his closest advisor and friend, Ian Makone, as one of his deputies, but such a move would anger other senior party figures.
As a further sign of the worsening internal division in Zanu-PF, Mugabe was last month forced to dismantle his grassroots leadership structures, called district co-ordinating committees, after factional battles erupted countrywide.
The new vice-president draft would also take away Mugabe's power to appoint provincial governors and weaken the authority of traditional chiefs. Governors would be chosen by the party with the most votes in a province. Governors and chiefs form part of Mugabe's patronage network, useful in the countryside as a result of their influence over the distribution of land and food aid.
The draft also proposes that Parliament, not the president, approves the deployment of troops. The president would also no longer be able to make appointments to the judiciary. Judges would be picked only after public hearings conducted by the Judicial Services Commission.
Zanu-PF is also opposed to dual citizenship, proposed reforms to the attorney general's office and the setting up of a Constitutional Court. The party has added that Zimbabwe's struggle history is not visible enough in the preamble.
Justice Minister Patrick Chinamasa, who negotiated the draft on behalf of Zanu-PF, said the party was auditing the draft "clause by clause, sentence by sentence and word by word".
Jonathan Moyo, a member of the Zanu-PF politburo and a key party strategist, has said of the draft: "If you ask what is it that is in the draft that raises problems, I am tempted to say everything. But if I say that it has 18 chapters, I can tell you that each of the 18 chapters has a problem."
On Monday, Mugabe said the refer-endum would be held only once all parties had agreed to a new draft.
"Once there is consensus, we will go for a referendum. It is our expectation elections will fall soon after," Mugabe said.
Both MDC factions said they would oppose any amendments proposed by Zanu-PF, setting the stage for yet another deadlock.
"If any political party doesn't approve the document we have produced there is nothing to take to the next stage," Chinamasa said this week.
Douglas Mwonzora, the spokesperson for Tsvangirai's party, said Zanu-PF should let the draft go to a referendum.
"We also have areas that we need to be tightened, but we have negotiated for [a] long [time] and should leave it to the people to decide," he said.
The draft is supposed to be put to a referendum before new elections. The government planned to hold the referendum by October, but this now seems unlikely as Zanu-PF digs in its heels.