Extensive research on voter persuasion has often yielded null results, leading political parties to focus on mobilizing their core supporters. These key supporters are theoretically essential for linking parties to the electorate. Yet, causal evidence remains limited regarding the effectiveness of real-world party messages in influencing their core supporters’ behavior.

To address this gap, we conducted six pre-registered, large-scale experiments in partnership with a major political party in Romania. The experiments surveyed nearly 400,000 core supporters (over half of the party’s votes in the 2024 European elections) during the 2024 Romanian European, presidential, and parliamentary elections. Based on more than 68,000 responses, our findings show that messages designed to appeal to ingroup or outgroup partisan identities have substantively small and statistically insignificant effects on voter mobilization. Self-reported and behavioral measures confirm these results. While strategic voting messages show relatively stronger mobilization effects, their overall impact remains substantively limited.

Our study provides causal evidence that political parties struggle to design messages that effectively activate their core supporters during campaigns. This challenges the assumption that parties inherently know how to mobilize their voters and highlights the need for further research into what strategies genuinely drive core supporter engagement, particularly in the context of underperforming party messages.