By George Georgiopoulos and Renee Maltezou ATHENS (Reuters) - Former Greek premier Alexis Tsipras urged supporters on Saturday to give him a mandate to complete the country's political transformation, as a poll showed his leftist Syriza party's lead slipping ahead of elections next month. Tsipras abruptly resigned last week, days after clinching an 86 billion euro ($97 billion) bailout package from Greece's euro zone and International Monetary Fund lenders, aiming to crush a rebellion by far-left lawmakers and tighten his grip on power. Hopes the lenders might soon resolve differences over how to tackle Greece's existing debt rose on Saturday, when IMF head Christine Lagarde told a Swiss newspaper a form of restructuring rather than outright forgiveness should enable the country to cope.