The leftist Syriza party of former Prime Minister Alexis Tsipras will challenge Greece's creditors on unresolved aspects of the nation's bailout but implement unpopular reforms it has already agreed, it said on Sunday. As opinion polls showed Syriza's lead eroding, its campaign programme reflected unease about the 86 billion euros ($96 billion) rescue package it will have to follow if voted back into power, pledging to battle creditors on some areas still up for discussion if it is re-elected. Tsipras resigned this month to pave the way for early elections, aiming for a stronger mandate to implement an EU/IMF bailout and rid his party of anti-bailout rebels.