The advent of new social media has facilitated new means of political communication, through which politicians can address the electorate in an unmediated way. This article concentrates on political actors challenging the establishment, for whom new media platforms such as Twitter provide new tools to engage in a ‘permanent campaign’ against dominant mainstream parties. Such opposition is ostensibly articulated most strongly by populist parties, which can be seen as the ultimate challengers to the (political) ‘elites’. By means of two often-identified cases of populism in the Netherlands (the radical right Freedom Party and left-wing Socialist Party), this study explores how populist party leaders use Twitter messages (tweets) to give form to their adversarial rhetoric in practice. Irrespective of the different ways in which the politicians utilised the medium, our study shows that Twitter can serve as a valuable source to study the oppositional discourse of populist parties, and (shifting) party strategies more generally.