Little is known about how legislature size affects the political mobilization of societal interests. I propose that legislative downsizing events increase the cost of campaigns, and thereby spur additional lobbying by organized interests that corral monetary resources efficiently. I examine how numbers of organizations with registered lobbyists changed in response to legislative downsizing events in three states. Using synthetic control analyses, I find that downsizing did not affect organization totals in Massachusetts or Rhode Island, but that Illinois’ Cutback Amendment precipitated a 25-percent increase in organized interests. Further tests disconfirm that monetary-based interests were most likely to mobilize anew after the Amendment’s implementation. In general, these mixed findings imply that changes in legislature size alone are insufficient for affecting interest mobilization but that other kinds of legislative reforms, such as the transition from cumulative to plurality voting that accompanied Illinois’ downsizing, may affect mobilization rates.