比較憲政工程的傳統智慧認為半總統制的次類型總理總統制比另一次類型總統國會制運作更為穩定，並且援引法國或葡萄牙等總理總統制國家運作成功的個案作為相關的佐證。然而，一些總理總統制的新興民主國家，其運作上卻出現政治不穩定的現象。本文的目的在於探究為何這些總理總統制國家會出現不穩定的原因和模式以及透過這些反面個案 (negative cases) 來搜尋總理總統制成功運作的關鍵因素。 過去的研究認為在總理總統制的運作當中，國會的政黨組成情況對於政治穩定有關鍵性的影響，同時也論證如果總統黨能夠進入國會的多數執政聯盟，對於政治穩定會有正面的幫助。本文針對東歐的總理總統制國家烏克蘭、羅馬尼亞之憲政運作進行分析，探討國會的政黨組成情況對於政治穩定的影響。本文觀察兩個案例後發現，即使總統黨進入多數的執政聯盟後，還必須觀察總統黨在多數聯盟內部是否具有主導地位才能進一步判斷國會的政黨組成情況對於政治穩定的影響。如果總統黨無法在執政聯盟內部取得領導的地位，而是由總理黨取得主導地位，則總統權力可能會受到較大的制約，容易引發總統和總理之間的衝突，政治運作較為不穩定。反之，當總統黨能主導執政聯盟，則總統權力受到制約的情況較小，總統與總理較不易發生衝突，政治運作較為穩定。

The conventional wisdom on comparative constitutional engineering argues, on the basis of the successful experiences of France and Portugal, that the operation of premier-presidentialism (one subtype of semi-presidentialism) has been more stable than the operation of president-parliamentarism (the other subtype of semipresidentialism). However, some newly democratized countries which have chosen premier-presidentialism have witnessed political instability. This paper aims to explore why these countries have suffered from political instability and to identify the crucial factors which facilitate the democratic workability of premierpresidentialism, by way of analyzing these negative cases. Past research claims that the constellation of political parties in the parliament is the key factor affecting political stability and also suggests that if the president’s party is included in the ruling coalition, then it can substantially contribute to the political stability of premier-presidentialism. After delving into the cases of Romania and Ukraine, this paper further argues that not only do we need to observe whether the president’s party is in the coalition, but we also need to examine whether the president’s party has a dominant position in the coalition. If the prime minister’s party, rather than the president’s party, takes the helm in the coalition, presidential power and authority may be constrained, thereby leading to the clashes between the president and prime minister and sometimes to serious political instability. In contrast, when the president’s party leads the coalition, conflicts between the president and prime minister are less likely, leading to greater political stability.