Why do some political parties flourish, while others flounder? In this book, Meguid examines variation in the electoral trajectories of the new set of single-issue parties: green, radical right, and ethnoterritorial parties. Instead of being dictated by electoral institutions or the socioeconomic climate, as the dominant theories contend, the fortunes of these niche parties, she argues, are shaped by the strategic responses of mainstream parties. She advances a new theory of party competition in which mainstream parties facing unequal competitors have access to a wider and more effective set of strategies than posited by standard spatial models. Combining statistical analyzes with in-depth case studies from Western Europe, the book explores how and why established parties undermine niche parties or turn them into weapons against their mainstream party opponents. This study of competition between unequals thus provides broader insights into the nature and outcome of competition between political equals.