Liberal democracies and their allies are facing a generational challenge from increased and evolving efforts by foreign actors to undermine public trust and degrade democracy. This article examines the problem of foreign interference with particular reference to the US midterm elections of 2018 as a case study, to draw potential lessons for liberal democracies in advance of future democratic processes. These lessons are centred upon five vulnerabilities to malicious actors, which – if exploited, either partly or wholly – can potentially degrade a democratic political system. The five vulnerabilities incorporate democratic institutions, election infrastructure and private industry. They also include individuals, and the core ideas that underpin democratic norms and values. We call these the ‘Five Is’. The paper outlines the challenges facing the integrity of elections for liberal democracies and fills out the concept of the ‘Five Is’. We note that the ‘Five Is’ are causally linked and overlapping. Having discussed the ‘Five Is’, we then look at the US 2018 midterms as a way to clarify and specify the ‘Five Is’ in practice. The paper then offers eight recommendations for policymakers to increase the resilience of electoral processes to such threats and attacks.