Theorists of contentious politics have been divided over the influence on agency behaviour of ideational or cultural factors on the one hand, and rational strategic calculations of opportunities on the other. Based on an Israeli nonparliamentary opposition movement to the peace process, this study presents an integrative approach in which ideational variables are elevated to the level of independent motivating forces in which the objectives of the movement are embedded. Argumentation at the strategic level provides the movement with opportunities for increasing its support basis beyond the natural constituency with which it shares ideology and, ultimately, for mobilizing protest. The case illustrates how a political context that limits activity actually mobilizes contention, while a context of more open opportunity can demobilize protest activity.