Ressler led the HDZ's list for the European Parliament election in May 2019. The HDZ won 4 seats and the highest number of votes with 22.7 percent, while Ressler won the highest number of preferential votes on the HDZ's list; 52,309 total votes (21.71 percent) which was the fourth-best overall score.
In 2017, Ressler led the Working group on the new Statute of the HDZ. Currently, he is a member of the HDZ's Statute Commission and Vice-President of the Party's Judicial Committee. He coordinated the creation of the HDZ's Program "Croatia for Generations" for the 2019 European Parliament elections.
In addition to his committee assignments, Ressler is part of the parliament's delegation for relations with China.[5] He is also a member of the European Internet Forum[6] and of the URBAN Intergroup.[7]