Objectives The 21st Amendment to the U.S. Constitution repealing national prohibition is the only amendment ratified by state conventions rather than state legislatures. The referenda held to select delegates for these conventions offer a promising source of data for identifying determinants of support for prohibition repeal. Methods We use various proxy measures to determine the importance of economic, political, and demographic forces in motivating support for electing pro‐repeal delegate slates to state ratifying conventions. Results Urbanization, per‐capita income, percentage of Catholics, and support for the Democratic Party were correlated with support for prohibition repeal. Neither the percentage of evangelical denominations, gender distribution, nor foreign‐born population appeared to significantly influence referenda returns. Conclusions This study confirms the conventional understanding regarding Catholic opposition to prohibition and higher income areas’ support for repeal. The findings also indicate that congressional backers of repeal were correct in calculating that submitting the 21st Amendment to popularly elected state conventions, rather than rural‐dominated state legislatures, would improve the chances of ratification. Moreover, support for repeal among Democratic congressmembers and party leaders appears to have extended to rank‐and‐file Democratic voters as well.