An illuminating investigation into why some parties evolve with their times while others fall behind. Around the world, established political parties face mounting pressures: insurgents on the Left and Right, altered media environments, new policy challenges, and the erosion of traditional strongholds, to name just a few. Yet parties have differed enormously in their ability to move with the times and update their offers to voters. This variation matters. While adaptation does not guarantee a party’s electoral success, the failure to modernize can spell its decline, even collapse, and create openings for radical and populist parties that may threaten the future of liberal democracy. Parties under Pressure examines why some parties adapt meaningfully to social, economic, and political transformations while others flounder, focusing especially on the fate of Western Europe’s Christian democratic parties. Matthias Dilling reveals the under-appreciated importance of party factions. While very high levels of factionalism are counter-productive and create paralysis, more moderate levels of factionalism help parties to adapt by giving visibility to fresh groups and ideas. Dilling draws on extensive archival research in Germany, Italy, and Austria, as well as evidence from France, Japan, and beyond. Taking a comparative-historical approach, Parties under Pressure sheds new light on parties’ varying records of adaptive reforms over more than seventy-five years.