The Citizens' Constitution Theory, formulated by Alan Cairns, provides a powerful explanation of the changes in Canadian politics during the 1980s. It tackles a research question that has far-reaching implications for the fundamental dynamics of Canadian political life—namely, how does constitutional change affect political participation? Cairns's thesis has gained widespread acceptance in the relevant Canadian literature, but the linkages between attitudes and behaviour at the core of the theory have never been subjected to systematic tests with attitudinal research data. The purpose of this investigation, then, is to make operational the Citizens' Constitution Theory and to evaluate the empirical support for it. The authors outline the central elements of the Citizens' Constitution Theory and discuss how Cairns relates the core concepts to each other. They then suggest that the same concepts and their linkages might also be explained by an alternative theoretical perspective that comes from one variant of New Politics Theory. The empirical section of this analysis uses recently collected survey results to mount three different tests of the two theories. In the first instance, the focus is on the question: how well do both theories predict each set of linkages that can be found in the Citizens' Constitution Theory? The second test treats both theories comprehensively, as causal models, and examines the empirical support for them using path analysis. The final section evaluates the generalizability of both theories. The main finding is that New Politics Theory provides as good an explanation—and by some standards, a better explanation for recent changes in the patterns of Canadian political participation than does the Citizens' Constitution Theory.