策略性投票與選舉制度：棄保效應的個案研究

Abstract

本論文針對單一選區相對多數決制下的行政首長選舉挑選個案進行研究，研究時間自1994年至2014年，聚焦於總統與直轄市長選舉，並挑選五個個案，分析棄保策略成功或失敗的背景與原因。在五個棄保個案中，發現存在棄保模式：第一，棄小保大，會形成國民黨與民進黨間兩黨對決，棄保較易成功；第二，候選人實力相當。若排名第二與第三的候選人實力相當，發動棄保策略，選票大量轉移的可能性比較低，所以棄保較不易成功。 研究發現幾個影響棄保成功與失敗的關鍵因素：第一、候選人民意調查排名。在三位主要候選人之中，如果國民黨候選人民調結果排名第一或第二的狀況下，棄保容易成功，反之棄保容易失敗；第二，民調資訊混亂。由於民調資訊過於複雜與混亂，選民難以判斷選票該如何轉移，棄保容易失敗；第三，候選人之間需存在可替代性。候選人本身支持度不足的狀況下，為求勝選僅靠競選策略來積極爭取不同陣營的選票，大部分狀況下，獲勝機率仍低。 雖然在單一席次相對多數的選舉制度下，當候選人多於兩位，便存在棄保投票的可能性，但並不是每次候選人或政黨打出棄保投票就會成功。候選人自身的支持度高低也相當重要，如果支持度太低，要操作棄保亦不容易成功。因此，與其操作棄保投票，更重要的關鍵在於候選人是否著重在基本面的經營，例如：政策的推出、施政表現、選民服務等，否則光靠棄保投票將 無法成功勝選。

This paper studies strategic voting of the five important elections in Taiwan from 1994 to 2014. These elections include three mayoral elections in Taipei City (1994, 1998 and 2006), one mayoral election in Kaohsiung City (2010) and presidential election in 2000. The major political parties include Kuomintang (KMT), Democratic Progressive Party (DPP), and the so-called pan-blue political parties that withdrawn from KMT while remain aligned to KMT. In these selected cases, despite of single-member district, if there are more than two candidates, it is probable to appear strategic voting, but it is not successful every time. The following are the key findings of this paper: 1.There are two patterns of strategic voting. First, strategic voting are successful in cases involving giving up a weaker candidate in favour of a stronger substitute. Second, strategic voting fails in cases which poll rates of target candidates are close. 2.There are three important factors that influence the outcome of strategic voting. First, the target candidates’ rank in opinion polls may determine the result of strategic voting. Second, inconsistant poll results during a campaign period may disrupt the decision making process of voters, which lead to fail strategic voting. Third, the result of strategic voting hinges on whether the target candidates are viable substitutes.