Self-reports are conventionally used to measure political preferences, yet individuals may be unable or unwilling to report their political attitudes. Here, we compared implicit and explicit methods of attitude assessment and focused our investigation on the populist-mainstream political divide. Ahead of the 2019 European Parliament election we recorded electroencephalography (EEG) from 82 future voters while they expressed their agreement with survey items on different political issues. An Implicit Association Test (IAT) was administered at the end of the recording session. Neural signals differed as a function of future mainstream or populist vote and of whether survey items expressed the narrative of a populist or mainstream party. The combination of EEG responses and self-reported preferences predicted electoral choice better than traditional socio-demographic and ideological variables, while IAT scores were not a significant predictor. These findings suggest that measurements of brain activity can refine the assessment of socio-political attitudes, even when those attitudes are not based on traditional ideological divides.