Multi-party single-member constituency electoral systems such as Great Britain’s are characterised by the political parties carefully targeting their local campaigns to maximise the number of seats won. They focus their activities on constituencies where either victory is likely or defeat is possible, which involves ensuring that residents are aware of the local context and the party’s electoral prospects there. But how do voters assess a party’s chances of victory locally; how aware are they of the local situation and to what extent does party campaigning change this? Analysis of British Election Study internet panel survey data shows that although many voters had an accurate assessment of each party’s chance of victory locally in 2010 many either over- or under-estimated its chances relative to the actual situation. Furthermore a substantial proportion changed their assessments as the election campaign progressed. Those contacted by a party during the various stages of the campaign tended to assess its chances of local victory as higher than those not contacted, especially if they also thought it had a good chance of winning the contest nationally, and such individuals were more likely to vote for the party than those not contacted.