* Obasanjo compares situation with brutal Abacha-rule
* Former leader says Jonathan promised not to seek 2nd term
* Presidency says claims are reckless and baseless
By Joe Brock
ABUJA (Reuters) - Nigerian President Goodluck Jonathan has been told it would be "morally flawed" for him to run for re-election in 2015, in a scathing letter from his mentor and former head of state Olusegun Obasanjo.
Obasanjo lays down a damning appraisal of Jonathan's leadership, chiding him on his performance on everything from corruption and insecurity to the economy, in the 18-page letter entitled 'before it is too late', seen by Reuters on Thursday.
The former president even likens the current situation to the tenure of General Sani Abacha, whose five years of military rule in the 1990s were marred by human rights abuses and the looting of funds from Africa's biggest oil producer.
Open criticism from a high-profile figure in the ruling party may encourage politicians to defect to a newly-united opposition and give greater confidence to potential rivals considering standing against Jonathan for the party leadership.
The presidency responded by saying the comments by Obasanjo were "reckless, baseless, unjustifiable and indecorous", adding that the letter had been deliberately leaked to the media.
The potency of Obasanjo's criticisms and the public nature of the spat add to a weight of embarrassment for Jonathan, coming days after another leaked letter, this time from the central bank, opened an investigation into missing oil funds.
Nigeria's Senate on Wednesday launched a probe into the letter to Jonathan from the central bank, which said the state-oil company NNPC failed to account for $50 billion of revenue from 18-months of oil sales.
Africa's second-biggest economy and top oil exporter is growing as an investment destination but reports of violence, corruption and political instability often tarnish its image.
General Obasanjo, who was president for eight years until 2007, was a strong influence in bringing Jonathan to power but their relationship has since soured.
The former president says Jonathan promised prior to his election victory in 2011 that he wouldn't run again. Jonathan has previously denied making such promises and he has not said whether he will contest the 2015 elections.
Jonathan's assumed ambition to seek re-election is a thorny issue in religiously-mixed Nigeria, where alternating the presidency between the majority Christian south and the mostly Muslim north has been considered an unwritten rule.
Jonathan, a southern Christian, was vice president and came to power when President Umaru Yar'Adua, a northern Muslim, died in May 2010, three years into his first term.
Jonathan's campaign in 2011 angered many northerners who felt it was still their turn to rule for what would have been Yar'Adua's second term. If Jonathan runs again in 2015 it could create further animosity.
"It would be fatally morally flawed" to run in 2015, Obasanjo, a Yoruba Christian from the southwest, said in his letter. He advised Jonathan not to listen to "sycophantic" advisers and to heed the warning signs in north Africa.
"When the consequences come, those who have wrongly advised you will not be there to help carry the can. Egypt must teach some lesson," referring to a revolution and upheaval there.
Obasanjo's own reputation was tarnished towards the end of his presidency by attempts to make changes to the constitution, which would have allowed him to run for a third term.
Jonathan has brushed off several corruption scandals and personal attacks in the past but opposition is mounting.
Last month, several powerful governors defected from Jonathan and Obasanjo's ruling People's Democratic Party and joined the All Progressives Congress, a coalition opposition formed in February which now poses the biggest threat to the PDP since the end of military rule in 1999.
The PDP still controls a strong majority in the national assembly and has the biggest power base and financial muscle.
Jonathan is also struggling to quell an insurgency from Islamist sect Boko Haram in the north and rampant oil theft in his home Niger Delta region, which is straining the economy.
Every Nigerian president has had to cope with insecurity, governance and human rights problems, and each has been hit by eye-popping corruption scandals. Obasanjo thinks the situation is worse than ever; Jonathan says it's on the way to recovery.
(Editing by Peter Graff)