The International Food Policy Research Institute (IFPRI) has been publishing some of its older books online for free in .pdf format. For example, the 1993 book on The Political Economy of Food and Nutrition Policies, edited by Per Pinstrup-Andersen, contains a chapter (.pdf) on the politics of the U.S. Food Stamp Program, written by Margaret Andrews and Katherine Clancy.
The chapter explores the reasons for the growth in the nation's leading food assistance program between the mid-1960s and the mid-1990s. What explains the Food Stamp Program's growth? Was it to dispose of agricultural surpluses? Was it vote-trading or "logrolling," in which urban legislators agreed to vote for farm programs in return for rural votes for food assistance? Was it a consequence of the Civil Rights movement and the growing democratic strength of the poor? My quick summary of the chapter: some of each explanation. But the details are interesting.