How does a political party become ‘mainstream’? And what makes some parties receive arguably the opposite designation – ‘pariah party’? This conceptual article examines the processes by which parties’ mainstream or pariah status must be constructed, negotiated and policed, not only by political scientists in the pursuit of case selection, but by several actors actively involved in the political process, including media actors and political parties themselves. It explains how these actors contribute to these processes of ‘mainstreaming’ and ‘pariahing’, considers their motivations and provides illustrative examples of how such processes take place. As such, the article moves beyond the literature on the ways in which mainstream parties seek to deal with or respond to threats from a variety of pariah parties, instead paying attention to how those parties have been constructed as pariahs in the first place, and how these processes simultaneously contribute to the maintenance of mainstream party identities.