Harare - Zimbabwean President Robert Mugabe's wife once said the 93-year-old leader should run "as a corpse" in the 2018 elections if he dies before the vote. Now politically ambitious Grace Mugabe is positioning herself as a possible successor, saying one of the ruling party's two vice presidents should be a woman.Her remarks on Thursday inject extra intrigue into a succession debate that has featured fighting within the ruling Zanu-PF party and a widespread sense of uncertainty in a country with debilitating economic problems.The world's oldest head of state, who recently returned from medical treatment in Singapore, is holding campaign rallies around the southern African nation but has slowed down considerably in recent months as his advanced age takes a toll.The ruling party should restore a provision in its constitution stating that one of Mugabe's vice presidents in the party should be a woman, said Grace Mugabe, according to state broadcaster ZBC.Her comments to a meeting attended by her husband also for the first time publicly challenged him to name a successor, wading into a subject that he has regarded as taboo.The ruling party had scrapped the clause requiring a female vice president in 2014 when the president fired Joice Mujuru, a former ally who was harshly criticized by Grace Mugabe, and replaced her with Emmerson Mnangagwa, the justice minister.If the clause is restored, it could mean one of the president's two male deputies should step down. Mugabe, who has been in power since independence from white minority rule in 1980, has traditionally appointed the ruling party's vice presidents to the same positions in government.Mnangagwa is the more prominent of Mugabe's deputies. The other, Phelekezela Mphoko, is from the minority Ndebele tribe and has criticised those who suggest he cannot lead because of his ethnic background.The first lady's comments appear to be a shift from her fierce defense of the president. Earlier this year she said that "if God decides to take him, then we would rather field him as a corpse" in the 2018 election.