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What impact do mass civil society groups have on public policy? We study this issue by analyzing opposition to national prohibition by German-American groups and associations in the early twentieth century, before and after state-sponsored suppression of them that coincided with U.S. entry to World War I. We measure German-American civil society and organizational strength across time and geography based on historical club directories, newspaper directories and petitioning activity. Comparing votes in the House of Representatives on two near-identical proposals for constitutional amendments—the defeat of the Hobson Prohibition Amendment in 1914 and the successful passage of the eventual Eighteenth Amendment in 1917—we find suppression mattered most in districts located at the middle of the German-American population distribution, where we hypothesize representatives were most persuadable. We estimate that without suppression of German-American organizations the Prohibition Amendment would not have received enough support for passage. Our findings add to an understanding of when and under what circumstances groups and organizations successfully influence public policy and provide a new explanation for the passage of the Prohibition Amendment.