The Japan Innovation Party (hereafter, IshinIshin, Japan Ishin no Kai (Nippon Ishin no Kai)) faced a notable decline in proportional representation (PRProportional representation (PR)) votes in the 2024 House of Representatives election. Why did many people who voted for Ishin in past elections withdraw their support in 2024? This study investigates the reasons for Ishin’s defeat in the 2024 election through empirical analyses based on two online surveys. Study 1 examined the relationship between the evaluation of party leader Baba and support for Ishin using an online survey conducted between September 30 and October 4 among eligible voters in Osaka. The results of Study 1 show that evaluations of Baba were significantly related to support for Ishin, even when controlling for its relationship with support for Osaka Ishin. Study 2 examined the relationship between evaluations of Ishin’s competence and changes in voting behavior from past elections, based on an online panel survey that was conducted before and after the 2024 election. The results of Study 2 demonstrate that evaluations of Ishin’s competence were strongly associated with whether individuals who voted for Ishin in past national elections continued to do so in 2024. These findings suggest that the decline in evaluations of Ishin’s competence was likely the primary reason for its defeat in the 2024 election.