本文探討台灣地區各主要政黨在區域立委選舉中，分裂與合作所可能造成的政治影響。嘗試由政黨的角度出發，以1983~2001年間七次區域立委選舉，國內各主要政黨的選舉提名、席次分配及選票動員情況為分析對象。透過有效動員指標的測量、提名失誤與席次紅利迴歸模型的建立，及實際表現與最適情況模擬差異估算發現：同屬泛藍政黨集團下的各政黨，確實因為分裂而有不利的影響，且泛藍政黨集團下的政黨分裂，主要受益者為民進黨。然而，泛藍政黨集團下的政黨分裂，及跨黨合作難以形成與落實的困境，除了受到選制本身存在有利分裂因素影響外；另一個更為實質的難題，即是泛藍政黨集團下各政黨，在選票瓜分上的極度不均等及穩定度的不足。所以，造成跨越政黨分際的合作難以形成，及落實的困境。而本文所進行的無失誤模擬，及所採用的最高可當選數估算方法，提供了SNTV選制下，政黨合作的新誘因，即透過反事實的模擬（counter-factual simulation）提供了新的資訊。

To identify the political effects of party splitting and cooperation, this paper uses two kinds of indicators, nomination error free and effect-tiveness of vote mobilization, to estimate the political effects of coopera-tion between pan-blue parties in legislative elections from 1995 to 2001. This research finds that if pan-blue parties had fully cooperated in these legislative elections, pan-blue parties would have gained more seat bonus-es. However, in reality, the pan-blue parties’ splitting has induced the loss of seat bonuses since the 1990s. The united DPP, on the other hand, has gained more seat bonuses, which can be attributed to the inter-party competition of pan-blue camp. however, through counter-factual simula-tion, this paper introduces the new information of motivating inter-party cooperation under SNTV electoral system.