In recent decades, there has been a considerable expansion of citizen participation in protests and voluntary advocacy groups. To analyze this development, the social movement literature and the interest group literature have emerged. Yet these two bodies of literature have not communicated with each other and have rarely incorporated knowledge from other fields in political science. As a result, critical questions remain unanswered regarding the political influence of advocacy groups. How do they affect politicians? To what degree do informal groups use lobbying tactics? Are socioeconomically advantaged groups more influential? This thesis endeavors to address the above shortcomings by bridging the literature on social movements, interest groups and political parties. The purpose of the thesis is to explain if and how advocacy groups affect public policy and to analyze which resources that are required to influence political decisions. The focus is on informal and loosely organized social movement organizations (informal SMOs): parental networks, staff networks, and village networks. To test my arguments, I use a unique database on protests and lobbying against school closures in Sweden. Closures of public schools have been one of the most important drivers of political activism in Sweden. The results are presented in three essays. Essay I tests new electoral mechanisms that could condition the political influence of advocacy groups. The results suggest that the political influence of informal SMOs on school closure decisions varies according to the type of voter they mobilize: swing voters or core voters. Essay II demonstrates how informal SMOs use lobbying tactics, such as presenting policy-relevant information, to influence politicians. Social movement scholars often focus on protests and ignore lobbying tactics. However, the results show that SMOs that present policy-relevant information are more likely to stop school closures than SMOs that mobilize large protests. Essay III analyzes which informal SMOs exchange policy-relevant information with politicians. Previous studies on the use of lobbying tactics have ignored activist resources. My results suggest that SMOs mobilizing high-income activists and activists with analytical and civic skills are more likely to present policy-relevant information. This is problematic given normative ideals of equal access to decision-making by all members of society.