Since 2009, the rise of the most important Belgian Francophone left-wing populist party, i.e. the Parti du Travail de Belgique (PTB), has been increasingly seen as a challenge for mainstream parties. Given the lack of research on Belgium within the field of political left-populist discourse, this paper analyses the effect of the growing popularity of the left-populist party on mainstream parties’ discourse. To investigate this issue, a discourse analysis has been conducted following the Modified Spatial Theory which argues that, when triggered, mainstream parties choose between three different strategies (accommodative, dismissive, or adversarial) to respond to the rise of populism. The findings show that, the rise of the PTB has had more effect on the Socialist Party’s discourse, which has accommodated and converged with the PTB on several typically populist issues, while the other two mainstream parties have rather dismissed and tried to discredit the political discourse of the PTB.