The aim of the paper is to understand the rise and subsequent marginalization of the Samoobrona (Self-defence) movement in contemporary Poland. The Samoobrona movement was founded in 1992 by a group of financially over-extended farmers who combined into a single entity two legally distinct organizations, one a trade union and the other a political party. It became famous in the nineties through the radicalism of its members during the farming demonstrations. To everyone’s amazement it then succeeded in assembling a broad electorate and becoming one of the largest political forces in Poland. Totalizing more than 10% of the vote at various elections from 2001 on, it even entered the administration for a short time, before being brutally marginalized after the early parliamentary elections of 2007. Using original research based on interviews with Polish politicians, media content, archival documents and statistics on party members, I examine the development conditions of such an « outsider » political group in a context of “transition” to representative democracy. Demonstrating the low analytical value of the concept of populism, I show that the double structure of the Samoobrona movement, at once party-political and trade-union based, is the key factor to understand its “successes” and “failures”. By allowing the Samoobrona movement to operate simultaneously on various arenas, it played a decisive role in its recognition as a major contender of the competition for the definition and the representation of social interests in post-communist Poland. Nevertheless, it also prevented the movement being seen as a legitimate actor by other contenders and hindered its institutionalization as a long-lasting political party.