In 2020 he succeeded Erik Tegnér [fr] as the head of the collective “Racines d’avenir”.
[2] He simultaneously held this position as a parliamentary attaché to Orne senator Vincent Segouin [fr] of the Les Républicains party.
He separated from him on 10 December 2021 when Casterman became involved in Éric Zemmour's campaign in the 2022 French presidential election.
[3][4] While he was deputy for school affairs in the commune of Englos, he stood for Reconquest in Nord's 11th constituency in the 2022 French legislative election where he obtained 3.36% of the votes cast.
[6] Although his candidacy was supported by the National Rally (RN), he defined himself as a miscellaneous right candidate.