The rapid adoption of marriage equality legislation for non-heterosexual individuals in Europe is attributed to many factors, including dramatic shifts in public opinion, the work of transnational activists and changing international norms. Usually, these factors must be filtered through the halls of parliaments where most policy change happens. Given the importance of parliamentarians’ attitudes, it is surprising that we know so little about how attitudes towards same-sex marriage are distributed across political candidates in Europe and what factors shape them. This article fills that gap by using an underutilised dataset on the political preferences of candidates for parliamentary office. We find that beyond attachment to party families, a candidate’s religiosity and practice has a greater effect on a would-be Member of Parliament’s attitudes towards same-sex marriage. The findings suggest that the success of parliamentary action on lesbian, gay, bisexual and transgender rights depends not on the partisan composition of the legislature, but rather on the representation of secular candidates.<br /><br />Key messages<br /><ul><li>Support among parliamentary candidates aspiring to become legislators is an important prerequisite for the advancement of lesbian, gay, bisexual and transgender rights in polities where institutional equality is brought about via legislative reforms.</li><br /><li>Over and above partisanship attachment and ideological positions, the religiosity of would-be Members of Parliament is important in establishing support for same-sex marriage.</li><br /><li>The secular composition of legislative chambers, rather than its partisan composition, is more likely to predict the success of the advancement of lesbian, gay, bisexual and transgender rights.</li></ul>