Why do parties offer broad or narrow policy agendas to voters? Taking up the call to focus more on ‘issue diversity’ in election campaigns, this article argues that agenda scope is informed by (1) parties’ experience with government participation and (2) their internal organizational structure. Non-governing challengers, which are losers in the current system, seek to change the political status quo by focusing on a few issues only, whereas mainstream parties have an incentive to reinforce existing patterns of competition and thus distribute their attention across a wide range of issues. However, the extent to which parties respond to these external stimuli depends on intraparty politics. Party leaders seek to satisfy vote- and office-seeking motivations and want to ‘appeal broadly’, whereas activists want the party to ‘speak to the base’ and narrow down its issue appeals. Analyses of party agendas in 18 European democracies (1950–2013) support these expectations.