Al Jazeera's correspondent in Harare, Supa Mandiwanzira said: "Many had expected that Zanu PF would go into this election as a divided party, president Robert Mugabe on one side, Simba Makoni on the other side.
"Just as is the main opposition, the Movement for Democratic Change, also split. Its got two formations that will be contesting in the election.
"There has been speculations that there are discussions with other opposition groups for all of them to unite as the presidential candidate," he said.
The party's constitution imposes the automatic expulsion of members who defy Zanu-PF resolutions made in December that chose Mugabe as its sole presidential candidate.
Mugabe has said he is ready to campaign against Makoni and the fractured opposition Movement for Democratic Change.
"I am very raring to go and raring to fly," the state-run Herald newspaper quoted Mugabe as saying.
On Monday, Mugabe chaired a politburo meeting to vet candidates standing for parliament and local council seats.
Polling for these seats will be held on the same day as the presidential vote on March 29.
On Monday a human-rights body said conditions for free and fair elections are not in place.
Party primary elections began two weeks ago and have been marred by allegations of vote rigging, favouritism and violence.
Last week, court officials in southern Zimbabwe charged five party members for public violence after they hurled stones at the car of Samuel Mumbengwegwi, a finance minister and close ally of Mugabe.
The minister dispersed the five by firing a hand gun into the air.
The five, who accused Mumbengwegwi of trying to disqualify a local councilman from the primaries, were later granted bail at a hearing two days after the confrontation.