In September 2010, Dijk became a member of the municipal council of Groningen as a temporary replacement for the departing Peter Verschuren [nl]. In January 2013, he succeeded Eelco Eikenaar [nl] as the leader of the SP group in the municipal council.[2] Among other achievenments, he strengthened the position of tenants living in poor conditions.[3]
Next to his council work, Dijk had also been active for the SP as a campaign leader and a member of the party board.[4] In the 2021 Dutch general election, he was listed tenth on the candidate list of the SP. The party only received votes for nine seats, and he was therefore not elected into the House of Representatives. In April 2023, Dijk joined the House of Representatives after Maarten Hijink vacated his seat, becoming the party spokesperson for health.[5] After taking office, he committed to preventing the closure of hospitals, such as those in Zutphen and Heerlen.[6]
Dijk succeeded Lilian Marijnissen as leader of the SP when she stepped down in December 2023 due to disappointing election results. In his first speech as party leader, Dijk said the party's focus would be on class struggle, opining that it is necessary to combat tension and division resulting from the dominance of neoliberal thought in the Netherlands since the 1990s.[7] Besides serving as the SP's parliamentary leader in the House of Representatives, he is the party's spokesperson for healthcare, finances, and Europe.[8]
^"Proces-verbaal verkiezingsuitslag Tweede Kamer 2021" [Report of the election results House of Representatives 2021] (PDF). Dutch Electoral Council (in Dutch). 29 March 2021. pp. 106–110, 231. Retrieved 21 December 2023.