Descriptive statistics of ruling party data.

This paper examines the relationship between ideological polarization and party disloyalty, focusing on the moderating role of the status of a political party in the legislature, i.e., the ruling party or the opposition party. It hypothesizes that the ruling party is willing to endorse disloyal candidates whose issue positions are not close to their own party’s platform, whereas the opposition party is likely to punish disloyal candidates to demonstrate party unity in the nomination process. The present study tests this hypothesis, using data from South Korea, where the nomination process for the parliamentary election is dominated by party leaders. The results are by and large consistent with the hypotheses. In line with previous studies, our results suggest that party loyalty is one of the driving forces of polarization in politics. In order to fully understand party polarization at the level of political elites, it is necessary to consider heterogeneous effects of party members’ behavior on candidate selection, varying across the party’s status, either the ruling party or the opposition party.