Mobilising support when the stakes are high:mass emails affect constituent-to-legislator lobbying

Mass emails are frequently used by advocacy groups to mobilise supporters to lobby legislators. But how effective are they at inducing constituent-to-legislator lobbying when the stakes are high? We test the efficacy of a large-scale email campaign conducted by the UK's main anti-Brexit organisation. In 2019, the group prominently displayed a ‘Write to your MP’ tool on their website and assigned 119,362 supporters represented by legislators with incongruent views to one of four email messages encouraging them to write to their MP or a control condition (no email). Messages varied across two factors: whether the MP's incongruent position was highlighted, and if urgency was emphasised. We find that 3.4 per cent of treatment subjects contacted their representative, compared to 0.1 per cent of those in the control, representing an additional 3,344 emails sent to MPs. We show that there was no substitution away from the most frequently used online legislator contact platform in the United Kingdom. While, on average, position and urgency cues had no marginal effects above the standard email, the most engaged supporters were more mobilised when informed that their MP held incongruent views. This study shows that advocacy groups can use low-cost communication techniques to mobilise supporters to lobby representatives when the stakes are high.