傳統的研究智慧中，總統制和兩黨制的配套被認為是一個民主國家較容易運作的組合。主要的理由是兩黨制較不可能出現意識形態的極化，以及因為兩個政黨需要贏得中間位置的選票，其可以促成政黨之間較溫和及向心的競爭，因而有助於總統制的運作。然而，近年來，一些兩黨總統制國家發生嚴重的憲政危機，甚至促成民主衰退。為何這些兩黨總統制國家會走上民主衰退的道路，是本文所要探索的研究問題。首先，本文透過對於拉丁美洲兩黨總統制國家的個案分析發現，憲政結構的因素如總統和國會的權力抗衡會影響兩黨總統制運作的成功或是失敗。本文經由對哥斯大黎加以及 1993 年 之前的委內瑞拉的個案說明，弱權總統對強權國會的憲政結構，較能促進兩黨總統制民主的成功運作。其次，從 1993 年到 1999 年的委內瑞拉和哥倫比亞的例子說明，強權總統對強權國會的憲政結構，促成兩黨總統制國家出現嚴重的憲政危機和民主衰退。本文的分析說明了總統制和兩黨制配套的情形下，強權總統和強權國會之權力抗衡會導致民主衰退。

Past research argues that presidentialism and two-party systems are workable combinations that can facilitate democratic stability. The causal mechanisms are that ideological polarization is less possible to appear in a two-party system, that two parties needing to win votes from the center encourages moderation, and that the absence of the extremist parties and the centripetal nature of party competition favor democratic stability. However, in recent years, some presidential countries with a two-party system in Latin America have suffered serious constitutional crises, thereby causing democratic decay. This paper aims to answer why these presidential countries with a two-party system have gone the way of democratic decay. After analyzing the cases of presidential countries with a two-party system in Latin America, this paper shows that the constitutional structure, especially balancing between the president and congress, affects the success or failure of operation of presidential countries with a two-party system. First, presidentialism in Costa Rica and Venezuela(1961-1993)illustrate that the constitutional structure, weak president versus strong congress, facilitates democracy in these two countries. Second, presidentialism in Venezuela (1993-1999)and in Columbia present that the constitutional structure, strong president versus strong congress, brings about democratic decay. This paper argues that the success or failure of democratic operation in presidential countries with a two-party system depends on the balancing of inter-branch power.