By Nick Tattersall
LAGOS (Reuters) - Nigeria's two main opposition parties are mulling an eleventh-hour alliance to try to unseat President Goodluck Jonathan in elections on Saturday, opposition sources said.
Officials from the two parties -- the Congress for Progressive Change (CPC) and the Action Congress of Nigeria (ACN) -- met in Abuja on Tuesday to discuss the possibility of fielding a single candidate against Jonathan, the sources said.
The deal could see former anti-corruption chief Nuhu Ribadu, the ACN candidate, step down to allow ex-military ruler Muhammadu Buhari of the CPC to lead the main opposition ticket, the sources said.
The ACN would field its vice presidential candidate Fola Adeola, a respected businessman, to run alongside him.
"There is a meeting on the electoral fortunes of the opposition parties but the details I cannot confirm ... The meeting was still on (late on Tuesday)," Buhari's spokesman Yinka Odumakin told Reuters.
A spokesman for the ACN could not immediately be reached.
The ruling People's Democratic Party (PDP) has won every presidential race since the end of military rule in 1999 and Jonathan is considered the favorite, but the opposition parties are hoping their regional strengths could force a run-off.
Buhari has strong support in parts of the mostly Muslim north while the ACN has its stronghold in the southwest.
There remain significant hurdles to any deal, not least selling it to the respective party faithful just days before the election. It is already too late to re-print ballot papers and any agreement could backfire by creating confusion.
"I don't know if everybody is happy with it. There may be time to complete a deal but not to sell a deal. It remains to be seen if this agreement can cross the finishing line," said one opposition source who declined to be named.
More than a dozen candidates are vying for the presidency, but Jonathan, Buhari and Ribadu are the main contenders.
Last Saturday's parliamentary polls, in which the ACN gained seats but the CPC failed to perform as strongly as expected in some areas, have further complicated negotiations.
Sources said one possible arrangement discussed prior to the parliamentary vote had been for the opposition party that won the most seats to field the presidential candidate.
That would now mean Ribadu leading the opposition charge.
But Buhari -- who ruled Nigeria from 1983 to 1985 -- is much more of a political heavyweight than Ribadu, the youngest of the main presidential candidates.
"There is no way Buhari would be persuaded to step down," said another source familiar with the negotiations.
The PDP -- which has dominated Nigerian politics since 1999 -- lost ground in the parliamentary vote, considered by observers to have been the most credible in Nigeria for decades. Based on results released so far, its majority in the lower house could slip to just over half from three quarters.
The opposition is hoping that momentum could carry it through to a strong showing in the presidential election and governorship votes in the 36 states a week later.
But the PDP has a national machinery where the main opposition parties remain essentially regional players. Forming a credible alliance to overcome that hurdle at such a late stage could prove difficult.
(Writing by Nick Tattersall, editing by Tim Pearce)