Why do some mainstream center-right and center-left parties in Europe maintain their support while others decline? This paper argues that the clarity of party positions on salient wedge issues is a key factor. Wedge issues often cross-cut the diverse and broad electorates of traditional mainstream parties. This creates internal divisions that make it difficult to adopt clear, unified positions. Ambiguity in a party’s stance increases the risk of electoral losses, while a clear position can stabilize or even bolster support when an issue is highly salient. Empirically, this paper relaxes the unitary actor assumption and examines the effects of intra-party division on political support across European countries, focusing on immigration, a wedge issue that has become increasingly salient with the rise of far-right parties. Using Natural Language Processing (NLP) on a corpus of party Tweets, parliamentary speeches and press releases in European countries, it measures variance and thus issue ambiguity in party positions on immigration and assesses the effects on national polling data, tracked monthly and, in some cases, weekly. The findings show that mainstream parties with greater internal ambiguity suffer larger declines in support, whereas those that deliver clear positions are more resilient. This study highlights a critical explanation for variations in mainstream party decline across Europe: their ability to manage internal divisions and articulate coherent stances on cross-cutting issues. It underscores the importance of message clarity in an era when issue salience, particularly around immigration, challenges the unity of broad-based parties and reshapes political competition.