This paper investigates the relationship between political party cooperation and Duverger’s Law. A theoretical model is build in order to demonstrate whether two party leaders would cooperate or not and therefore nominate only one candidate to campaign. The results shed light on how Duverger’s law is satisfied through pre-election coordination between political parties. Under three assumptions, Duverger’s law is satisfied. Otherwise, candidates of three political parties will run election under the plurality electoral system.

In addition, Kuomintang (KMT) party and People First (PF) party decided to campaign 2004 presidential election together. Lien Chan and James Soong are recommended as the presidential and vice-presidential candidates by pan-blue coalition. This is the first time in Taiwan electoral history that political parties formally form an alliance. Therefore, Chen Shui-bian and Lien Chan will run the election and therefore becomes the competition between pan-green coalition and pan-blue coalition. The above theory can analyze this case and explain why such a party cooperation exists as well.