HARARE (Reuters) - Zimbabwe Electoral Commission (ZEC) chairwoman Rita Makarau resigned on Friday, months before a vote whose credibility is crucial to the new army-backed government s efforts to re-engage international lenders and lure investors, a senior government source said. Makarau, seen as an ally of 93-year-old former president Robert Mugabe, gave no reason for her resignation, the official, who declined to be named, said. President Emmerson Mnangagwa, who was sworn in two weeks ago in the wake of the de facto military coup that ended Mugabe s 37-year rule, pledged to hold elections as scheduled next year. Finance Minister Patrick Chinamasa mentioned the credibility of the elections at least five times in a budget speech on Thursday, a sign of the vote s importance in shoring up Harare s democratic legitimacy. Opposition parties have demanded reforms to an electoral system they say is skewed in the ruling ZANU-PF party s favor. Makarau, who has been accused of being partisan, was overseeing an overhaul of the voters  roll, which the opposition Movement for Democratic Change has described as  shambolic . Makarau did not answer her mobile phone. A spokesman for the ZEC was unable to confirm Makarau s departure. 