This study analyses the impact of institutional change upon political behaviour. Beginning in 1968 a series of reforms transformed the American presidential nominating process, amounting to the most substantial changes since the inception of national conventions. This study seeks to comprehend the effects of this transformation upon the strategies employed by candidates and in so doing assessing the influence upon the nominating process of various actors - party leaders, voters, interest groups, campaign organisations. The comparative method is adopted to elucidate the impact of change. The content and execution of strategy is compared across continuous periods, the nominations immediately before (1960 - 68) and after (1972 - 1980) reform. A party function, candidate selection, is set within a theoretical discussion of party. Familiar models of party are examined and criticised for their inapplicability to the American case for their omission of an intra-party role for voters. An additional ideal type model is developed of a party dominated by. voters. -; the application of direct democracy to intra-party affairs. This additional type is integrated into the schemes analysed earlier, increasing their relevance to American practice and providing a set of logical, possibilities against which party reform can be measured. Previous reforms of the presidential nominating process are described and recurrent trends identified. The background to the impetus for reform originating in the discontents of the McCarthy campaign-to mobilise voters into the party-dominated selection process is described. The composition and functioning of the Commission on Party Structure and Delegate Selection authorised in 1968 to recommend reform proposals is discussed. The implementation of reform, its unintended consequences, the work of subsequent reform commissions in the Democratic Party, change in the Republican Party and innovations in the regulation of campaign finance are detailed. The combined impact of these reforms transformed the context of nominating campaigns. Primaries became the dominant delegate selection mechanism. The non-primary process was opened to extensive voter participation. In both processes the linkage between the candidate preferences of participating voters and the resulting delegates tightened. The size of campaign donations was limited, federal funds became available, and ceilings were placed on total expenditures for recipients of federal aid. Having depicted the altered context of the nominating contest the study analyses the content and execution of strategy in the two periods. The basis for comparison include the choices of strategies, the form of campaign organisation and their relations with party organisations, the conditions of interest group influence, the role in strategy of the primary and non-primary processes, the content of candidates' appeals and the means employed to communicate the campaign. The conclusion re-states the principal strategic differences between the two periods. The strategic consequences of reform are linked to the effects of previous reform efforts, and the model of parties developed earlier.