ABUJA (Reuters) - Nigeria s Atiku Abubakar, a former vice president and key ally of President Muhammadu Buhari, quit the ruling party on Friday saying it had  failed the people , and a spokesman said he might run in a presidential election due in February 2019.   Abubakar is the first political heavyweight to signal a likely bid for the presidency, which could pit him against 74-year-old Buhari, who took power in 2015 but has been absent for much of this year due to illness. Questions over Buhari s fitness or willingness to run in 2019 are rife in Nigeria, both within and outside his ruling All Progressives Congress (APC) party, which Abubakar had earlier quit in an unprecedented public rupture. Divisions within the party have been exacerbated by Buhari s recent long absences abroad for treatment for an unexplained ailment and discontent with his administration. The main opposition is also struggling to maintain unity, according to party members. In a statement announcing he was leaving the party, Abubakar said the APC  has failed and continues to fail our people  and accused it of  a draconian clampdown on all forms of democracy within the party and the government . A spokesman later told Reuters that Abubakar, who was Nigeria s vice president from 1999 to 2007 and has made numerous unsuccessful bids to become the country s leader,  is prepared to run for the presidency in 2019 .  An APC spokesman declined to comment on Abubakar s departure. 