總統是否應該兼任黨主席，在台灣曾經引起多次的爭論，雖然正反意見存有相當歧異，但其實不管是國民黨或者是民進黨，在其執政任內，都沒能抗拒總統身兼黨主席所帶來的黨政領導的便利。總統身兼黨主席真的能夠使政府的施政更為順暢嗎？顯然這些選擇兼任黨主席的總統是如此認為，才會做出這樣的決定。然後，在理論上，既有文獻多半忽略這項因素的重要性；在實務運作上，我們也看到總統在兼任黨主席之後，政府提案未必能在立法院通過。究竟總統兼任黨主席對於政府提案通過的影響為何？平均而言，是正面的影響並且達到統計上的顯著水準嗎？這些問題是本文分析的主題，分析個案則鎖定在陳水扁執行時期。本文認為，在這個時期裡，民進黨雖然沒有掌握立法院過半的席次，但是在其他條件相同的情形下，總統透過兼任黨主席，能夠使政府提案的通過率相較於總統未兼任黨主席時，由於總統不用透過代理人來確保他的意志在執政黨裡獲得貫徹，因此能夠使得黨意更為貼近總統意志，而且也有助於總統透過領導黨來領導國會，並使得黨政之間維持較佳的聯繫，促進黨籍立委對於政府立場與政策的支持與了解。其次，總統兼任黨主席能對反對黨形成壓力，使其減少對政府提案採取全盤反對的可能性，原因有二，一是因為總統兼任黨主席有助於執政黨團結一致支持政府政策，從而能夠更有效地爭取民意支持，並且讓反對黨感受到比較大的壓力；二是因為總統兼任黨主席將更能確保執政黨的政策偏好彺全國的中間選民移動，因此比較能夠贏得民意以及反對黨的支持。本文的理論預期也得到實證資料的支持。

Whether a president should also serve as a party chairman has been a controversial issue in Taiwan. However, both the Kuomintang and the Democratic Progressive Party nodded in agreement on this issue as long as they were the ruling parties. Does the fusion of the two roles really make the passage of government bills easier? Most of the existing literature neglected the important factor of a president acting as a party chairman, and hence provided no answer to this question. In actual practice, we observed several cases of failures in securing the passage of government bills in the Legislative Yuan even after a president was named the chairman of the ruling party. In this article, we argue that, other factors being equal, the fusion of the two roles does enhance the probability of the passage of government bills. In our arguments, two reasons explain why this relation holds. Firstly, when a president acts as the party chairman, he/she does not need to rely on an agent to help him/her achieving policy goals, and can ensure the party’s policy preferences are exactly the same as his/her own. A president acting as a party chairman can also directly lead the party in the Legislative Yuan and better ensure support from the party's legislators. Secondly, a president acting as party chairman also helps induce a more cooperative stance among the opposition parties because of the following reasons. The first is that a more cohesive ruling party under a president's leadership as the party chairman is more likely to gain public support, which in turn makes opposition parties more likely to adopt more cooperative stances on some government bills. The second is that a president acting as a party chairman will help his party policy stances move closer to those of the median voter, and these policy stances will be more likely to gain public support, inducing opposition parties to follow suit, at least on some government bills. The empirical findings provide supportive evidence for the hypothesis of this article.