Dominant visions of modern representative democracy posit that parties focus on contrasting issues during campaigns, leading to a diverse political supply. However, there is remarkably little empirical evidence to back up those claims. We argue that parties have little incentives to leave potentially rewarding issues to rivals. Lacking knowledge about their electorate and its short-term preferences, parties will monitor competitors and take up issues from each other. Empirical analyses covering all policy issues in a unique set of seven diverse advanced democracies over four decades are consistent with our predictions. Issue attention appears to be mostly inspired by rivals’ emphases, resulting in “tunnels of attention”. The model holds when introducing the alternative explanation of responsiveness to voters, corroborating the endogenous nature of tunnels of attention. Our new perspective has wide-ranging implications for party competition and representation.