The phenomenon of populism, which has been widely studied by the social sciences, is also put to activist, normative and often disqualifying use. As a result, it becomes an instrument of confrontation. In this agonistic context, populism is often an expression of the extreme right, the "neither right nor left" (M5S in Italy and "Gilets jaunes" in France), and suggests a violation of classical party divisions and structures and of the institutionnal consensus. A "left-wing populism" has emerged, asserting a post-Marxist ideology and philosophy while challenging the routine political order and legitimising alternative party pratices and structures. Here, we analyse the reception of "left-wing populism" arguments through extracts from the writings of its theoreticians, combinations of academic and ideological language, including Chantal Mouffe and Ernesto Laclau. We go on to show that certain movements and parties, including LFI in France, have shaped an identity as advocates of left-wing populism by appropriating these arguments and claiming to be "populist". Finally, external perspectives on the part of competing parties highlight partisan criticism of the populist left. Hence, we shed light on a discursive strategy of delegitimisation, expressed in particular by the idea of a "objective alliance of populists" from the extreme right and extreme left. By doing so, the study reveals the social construction and partisan uses of this concept, as well as their limitations.