Lagos - The leading opposition parties agreed Saturday to merge into an electoral behemoth they vowed would defeat President Goodluck Jonathan in 2015 polls.
Two parties, the Congress for Progressive Change (CPC) and the All Nigeria People's Party (ANPP), approved the move in separate conventions aired live on television.
A third, the Action Congress of Nigeria, okayed the mega party project last month.
The result of the merger between Nigeria's top three opposition movements will be called the All Progressive Congress (APC) if its registration is approved by the electoral body.
"This merger will make us too strong for the PDP to resist," said CPC leader Mohammadu Buhari during a convention attended by 3,000 delegates in Abuja.
"We must explore all oportunities to save our country," said Buhari, a retired general and former military ruler who has contested all presidential elections in recent years.
Buhari, a Muslim who was ousted in 1985 after two years in power, is seen as a possible candidate for the new mega party in the 2015 vote.
"We have to change Nigeria for the better and stop the endless drift that has been going on for 14 years the PDP (People's Democratic Party) has been in power," Buhari said.
Previous attempts to unite the opposition against the ruling PDP have failed but Buhari urged the new alliance's leaders to set aside their differences.
"It's time to sacrifice everything, time and resources for the good of the nation. The PDP government has failed in almost everything," he said.
"The only way to save our nation is for the progressives to merge and oust the PDP," he said.
While he is widely expected to run again, Jonathan, a southern Christian, faces a challenge from within his own party, including from those who believe he should stand aside in favour of a northern Muslim.
Buhari also accused the ruling party of failing to stem the deadly violence caused by Islamist group Boko Haram in the north.
"It has failed to secure the nation... there is mass suffering, widespread poverty and unemployment in the country," he said.
The ANPP held its convention in the northern town of Gusau and also vowed the new party would be too strong for Jonathan's party.
The ruling PDP has won every presidential election since Nigeria returned to civilian rule in 1999.
The 2011 election won by Jonathan was seen as a major improvement over previous polls marred by violence and rigging, though significant problems remained.
While the PDP has controlled the presidency, opposition parties have been gnawing away at its dominance at the state level.
If approved, the new APC party would significantly change the political landscape in Africa's most populous nation and top oil producer.
Some Nigerians view Buhari as an uncompromising opponent of the rampant corruption that has dogged Nigeria for decades, though he has also been accused of major rights abuses during his time in power.
Picking a presidential ticket could prove the toughest test for the cohesion of the new alliance.
In Nigeria, roughly divided between a mainly Muslim north and mostly Christian south, analysts believe a successful ticket requires a member of each faith in order to have cross-nation appeal.
The southwest is considered ACN's stronghold, where it holds the governorships of six of Nigeria's 36 states. The CPC and ANPP are stronger in the north.