While it is common to talk about political parties as unitary actors, parties generally harbor members with disparate opinions on important issues. In this chapter we describe and explain the varying level of ideological cohesion – the degree to which party members agree on political issues – in Latin America.

Cohesion is an important indicator of the nature of a party and, by extension, relations between that party and other political actors in the system. Foremost, cohesion affects the chain of relations between voters, parties, and legislators. If parties are cohesive, then voters can focus their attention on national party platforms and campaigns, safely assuming that the parties' local representatives will support the same objectives. This conception excites scholars concerned with the personal vote or party “nationalization.” (APSA 1950; Cain, Ferejohn, and Fiorina 1987; Morgenstern and Swindle 2005). Party cohesion also influences interparty negotiations by determining whether some or all of a party's members are potential coalition members for particular parties or the executive. Within a party, cohesion helps determine the willingness of backbenchers to entrust their party leaders with delegated authority (a defining trait of conditional party government; Aldrich and Rohde 2001). It also influences the party's ideological placement. Cohesion, then, is a defining factor in explaining patterns of legislative activity.

In this chapter we focus on cohesion as an indicator and facilitator of programmatic structuration.