Recently there has been a renewed interest in the role of local campaigns and their effectiveness on increasing turnout and support for political parties. However, there is a long-standing debate over the best way to measure campaign effort. This article advances the current literature by using a latent variable modelling approach to utilize, for the first time, evidence frorvey of agents, official records of campaign spending and individual voter survey data to produce a combined measure of campaign effort. This measure (latent variable) is then used in a structural equation model of party performance to assess the effect of the campaign effort of the three main parties at the 2005 British general election. In terms of both the delivery and effectiveness of campaigns the parties are found to behave in a way consistent with a rational model of party behaviour, though constrained by contextual factors.