Recent decades have witnessed a steady increase in women's political representation across most advanced democracies. The European Parliament (EP) has been no exception to this trend, with the share of female MEPs surging from 27.5% in 1999 to 40.\% in 2019. While increasing scholarly attention has been paid to understanding the determinants of women’s electoral success by either focusing on the supply or demand for female candidacy, research accounting for both is rare, especially in a cross-national setting. Hence, we do not know to which extent party gatekeepers or changing voter attitudes explain the increase in women's representation and the remaining inequality.

We address this gap by using a purpose-built multi-layered data set, including all candidates running in four EP elections (1999-2014). The analysis examines how both party-determined candidate viability and voter attitudes towards gender equality affect individual candidates' chances to win elections. The data permit analyses from a longitudinal perspective, and the institutional context of EP elections allows us to further examine how different types of PR electoral systems affect women's representation. Our results suggest that while women, overall, are as successful as men at the electoral stage (and voters holding more gender-equal attitudes helps), female candidates’ electoral success is particularly sensitive to party decisions at the nomination stage.