What explains contemporary numbers of interest groups in America? To answer this question and help address conflicting narratives in research, I examine the rise of interest groups in the states. Assembling an original dataset based on archival and secondary sources, I find that relatively few groups lobbied legislators prior to the 1960s or 1970s. During those decades, numbers of interest groups began to grow rapidly. I find that increases in lawmaking activities present inconsistent effects on the political mobilization of groups but increases in spending are strongly correlated with mobilization. In additional tests, I find that the effects of spending on group numbers vary by state and are not discernible in most states. In general, a historic transformation of state governments helps to account for the growth of state lobbying. Interest groups have remained active in state capitols ever since.