This paper regards Taiwan's 2009 local (i.e., county magistrate) election as a kind of midterm election and explains why the ruling party Kuomintang (KMT) got defeated by the opposition Democratic Progressive Party (DPP) from two theoretical perspectives-namely, the “lack of mobilization” theory and the “swing voter” theory. By using both aggregate-level voting records and individual-level survey data, our empirical findings are three-fold: first, in general, the DPP successfully mobilized their supporters to get out to voting in the 2009 local election while the KMT failed to do so. Second, previous KMT supporters in the 2008 presidential election were somehow reluctant to continuously support KMT candidates in the 2009 local election. Specifically, in the KMT advantageous county, such as Taoyuan County, previous KMT supporters were less likely to turn out to vote than those previous DPP supporters. On the other hand, in the DPP advantageous county, such as Yunlin County, a significant proportion of previous KMT supporters actually turned out to vote for the DPP incumbent candidate. Third, The conventional wisdom suggests that the performance of the ruling party usually becomes an important factor that affects voting behavior in midterm elections. Our analysis partially confirms such “referendum voting model” in the sense that some Taiwanese voters, particularly those who voted for the ruling party in the previous national election, may took into account the performance of the central government when casting their votes in the next election, even if it was just a local-level election.