The correct answer is "pressure groups".
A pressure group is an organized group of people, which could well be a formalized institution, which tries to persuade a government in order to curb public policy according to a particular interest it has.
Pressure groups include labor unions, NGOs, and nonpartisan organizations. These groups could decide to support a particular candidate during elections with the expectative that if he or she wins, public policy changes will be made in order to benefit their causes.