Downs的理論是意向研究途徑和理性抉擇模式的代表性理論。Downs所建構的兩個理論，第一個稱為「理性投票行為理論」，用來解釋選民的投票行為；第二個理論稱為「競爭空間理論」，用來解釋政黨的競爭策略。Downs認為選民和政黨都是「經濟人」，政黨從事選舉競爭，最主要的目的就是贏得選舉勝利，以取得執政的權力。政黨最主要的競爭策略就是在政治（意識形態）光譜上的移動。Downs認為在兩黨制或兩大黨競爭的情況下，因為大多數選民都在兩大黨的中間，因此兩大黨都會向中間移動，採取「向心競爭」的策略，有些學者稱之為「中間選民定理」。本文最主要就是在檢證這個定理。本文研究發現，從解嚴後到2000年政權輪替，國民黨、民進黨兩大黨確的向政治光譜的中間移動，採取「向心競爭」的策略，符合Downs的「中間選民定理」；但是從2000年以來至今，雖然大多數選民仍位居在政治光譜的中間，國、民兩黨卻呈現了「離心競爭」的狀況，特別是民進黨在2000年至2008年執政期間，向左移動的趨向特別顯著，顯然不符合Downs的理論。Downs的「競爭空間理論」主要假設，也許可以修正為：「在兩黨競爭下，兩大黨為了爭取多數中間選民的支持，在選舉前夕通常會短暫地向政治光譜中間移動，但是選後通常會很快回到原本意識形態的位置，也可能繼續朝離心的方向移動，而且執政黨移動的速率和距離通常會超越反對黨。」

Downsian theories are the representative theories derived from the intentional approach and rational choice model. Downs constructed two theories: the first is called the ＂theory of sophisticated voting＂ ich is used to explain the electorate's voting behavior; the second is called the ＂spatial theory of competition＂ which is used to explain the competitive strategies of political parties. Downs assumes that both voters and parties behave like an ＂economic man.＂ The main purpose for a party to enter into electoral competition is to win the election and ruling position. The main competitive strategies of parties are the maneuvering along the political (ideological) spectrum. In a two-party system (or under two-party competition), Downs argues that the two main parties will both move toward the center of the spectrum because most voters are located in the middle, between the two parties. This is called ＂centripetal competition＂ or the ＂median voters theorem.＂ The main purpose of this paper is to review and verify this theorem.Research results of this paper find out that from the lifting of martial law (1987) to the first rotation of power in 2000, both two main parties had adopted ＂centripetal competition＂ strategies and moved toward the ＂center (middle) of the spectrum＂ which had confirmed the ＂median voters theorem＂ and the ＂spatial theory of competition.＂ However, from 2000 until now, despite the fact that voters are still located in the middle, the major two parties-the KMT and the DPP-have adopted strategies of centrifugal competition. This is especially true of the Democratic Progressive Party's rule, from 2000 to 2008 where the DPP demonstrated a strong tendency to move leftward.The case after 2000 apparently disconfirms Downsian theories. The main hypothesis of Downs' ＂spatial theory of competition＂ perhaps can be revised as the following: Under two-party competition the two major parties, in order to win the support of most median voters, usually move toward the center temporarily before the election, but will quickly move back to their previous ideological position and may continue to move centrifugally, away from the center. In addition, the ruling party's rate and distance of the centrifugal move usually exceeds the opposition party's.