This paper compares the spread and impact of new digital modes of voter mobilization with more traditional methods (phone, mail and in person canvassing) in recent national elections in the US and UK. We develop hypotheses regarding the relative effects of online contacting and test them using election study data. Our findings show that while online contact is generally less frequent than the offline form in both countries, this gap is particularly pronounced in the UK. US campaigns also reach a much wider audience than their UK counterparts. In terms of impact, while offline forms remain most effective in mobilizing turnout, online messages are important for campaign participation, particularly among younger citizens when they are mediated through social networks.