How strong are the links between people’s different social identities and their political identities? I use a novel measurement design to elicit the relative extent to which citizens perceive social, cultural, and economic characteristics are relevant to their own identity and to their political commonalities with others. The experiment was carried out on the pool of regular respondents of the British Elections Study (BES), weighted on several demographics to be representative of England. It consisted of presenting two profiles, randomly selected from the 2017 post-elections BES, which included several demographic attributes, and then asking respondents to choose, in terms of politics, with which of the two they perceived they had more in common. Combining the respondent’s implicit trade-off choice and her self-categorization allows me to measure the relative strength of the individual’s social identities. The study shows an important role for ethnicity, noticeably surpassing different measures of social class, as the main characteristic for perceived political commonality. Additionally, I find that the relative primacy of ethnicity is not evenly distributed in the population, but aligned with voting behaviour. Specifically, it is the relevance placed on this aspect by Conservative and Leave voters in the 2017 general elections and the 2016 EU referendum that explains its overall strength.