Abstract: Just before the European election in May 2019 a YouTube video titled The Destruction of the CDU (Rezo, 2019a) caused political controversy in Germany. The video by the popular German YouTuber Rezo attacked the conservative Government party CDU (Christlich Demokratische Union Deutschlands) mainly for climate inaction. As a reaction to the subsequent attacks on Rezo and his video from the political establishment an alliance of popular German YouTubers formed to release a second video. In this video, the YouTubers asked their followers not to vote for the Government or the far-right parties, because they would ignore the expertise of scientists and the scientific consensus on anthropogenic climate change and therefore be unable to provide sustainable solutions for the future. This debate started as a YouTube phenomenon but quickly evolved into a national public discussion that took place across various social media channels, blogs, newspapers, and TV news, but also e.g., in discussions in schools, churches, as well as arts and cultural events. The focus of this contribution is on the formation of the heterogeneous coalition that emerged to defend and support the YouTubers. It prominently involved scientists and scientific expertise, but other forms of expertise and ‘worlds of relevance’ were also part of this coalition. The conceptual tools of ‘networked expertise’ and ‘ethno-epistemic assemblages’ are employed to explore expertise and credibility as well as the associations and networks of actors involved which illuminate how a single YouTuber was able to contribute to the unleashing of a national debate on climate change policy.