Mass antineoliberal protests in Greece that challenged the long-standing two-party system and austerity policies associated with both major parties (PASOK and New Democracy) were followed by the emergence of challenger party Syriza. Data gathered during interviews with figures from Syriza as well as other leftist parties and activists highlight, however, that the outsider-to-insider party-building process culminated in a delegitimized and fragile Syriza. The electoral strategy of Syriza leaders in conjunction with opposition pressures encouraged a moderation of the party’s brand. To pursue such moderation required insulating the leadership clique from radical voices via degenerative factionalism and oligarchization processes. Moreover, weak popular sector organizations were unable to counter the moderation-factionalism-oligarchization process. The culmination of the party-building process saw a Syriza that lacked a coherent brand or any societal connections, leaving it vulnerable to replacement in the party system.