Simba Makoni, a former Zimbabwean finance minister who plans to challenge Robert Mugabe for the presidency, has won the backing of a key politician.
Dabengwa's defection is a blow to President Mugabe
Former Interior Minister Dumiso Dabengwa is the first Zanu-PF political heavyweight to endorse Mr Makoni's bid.
Speaking at the launch of Mr Makoni's campaign for the 29 March vote, he said it was time for a new leadership.
Mr Makoni's entry makes the election a three-way contest that also involves opposition leader Morgan Tsvangirai.
President Mugabe has called his opponents "witches" and "charlatans". He also called his rivals "traitors and two-headed creatures".
Launching his re-election campaign on Friday, he said he was certain of victory in the presidential and parliamentary polls later this month.
Mr Mugabe has been in power since the country gained independence from the UK in 1980.
'For the people'
Former parliamentary speaker Cyril Ndebele joined Mr Dabengwa at the rally with Mr Makoni in Zimbabwe's second-largest city of Bulawayo.
"It is time they give way to a new leadership that can face up to the challenges facing our country," said Mr Dabengwa, who served as interior minister under Robert Mugabe.
Robert Mugabe is hoping to secure his sixth term in power
Mr Dabengwa was head of intelligence with Joshua Nkomo's forces during the liberation war in the 1970s.
Soon after Zimbabwe's independence in 1980, he was charged with treason - by President Mugabe's government - but acquitted.
In recent years, he has been a senior member of Zanu-PF's powerful politburo.
Addressing the crowd, Mr Makoni said: "I am not against Mugabe, I am just for the people."
"We are in a movement for renewal, revival," he said, adding: "There are still many in Zanu-PF and some in the MDC who are also going to come out and support me."
The economy has collapsed under Mr Mugabe's rule, with the annual inflation rate at more than 100,000%.
He blames the state of the country on Western sanctions imposed following allegations that he rigged the 2002 polls.
The sanctions are targeted at Mr Mugabe and his close associates - they are subject to a travel ban and an assets freeze in the European Union and the US.
As well as hyper-inflation, there are food and fuel shortages and just one in five adults is believed to have a job.