milan leader italy main opposition party say keep option referendum euro open event party elections fail convince brussels need change euro zone economic rule comment make state tv sunday luigi di maio man widely tip candidate prime minister anti-establishment 5-star movement say want negotiate concessions eu governance succeed europe change need referendum euro otherwise ask italians want stay say early month di maio tell reuters 5-star totally withdraw idea referendum euro call last resort employ possible ease eu governance rule comment come weeks parliament expect dissolve ahead elections early next year look unlikely produce clear winner 5-star movement predict emerge large single party next parliament repeatedly rule join coalition italy introduce new electoral system expect handicap 5-star favor instead mainstream political blocs italy 5-star party keep option euro referendum open