Are legislators responsive to the issue priorities of their constituents? E-petition systems have been adopted in many countries in recent years, commonly with the goal of strengthening the bond between representatives and their constituents. Despite this, we know little about the effects of such petitions on legislative behaviour. In the UK, signatures to government e-petitions provide MPs with information regarding the salience of different issues amongst their constituents. I examine whether increased local support for an e-petition affects an MP’s propensity to debate the petition in parliament, and the likelihood that they speak in favour of the petition. I introduce new data on e-petitions and propose a new strategy to control for MPs’ general tendencies to participate on similar topics in non-petition debates. Conditioning on the topic under discussion, I show that local support for an e-petition increases the probability that an MP participates in parliamentary debate relevant to the petition, and makes the MP more likely to speak in favour of the petition. However, these effects are conditional: online advocacy is only effective when addressing topics that are not strongly linked to the main dimension of partisan competition. When petitions address party-political issues, signature rates have no effect on MP behaviour. Finally, these effects are stronger for MPs in electorally competitive constituencies, and weaker for MPs who hold government positions.