Entryism

Entryism (also referred to as entrism or enterism, or as infiltration) is a political strategy in which an organisation or state encourages its members or supporters to join another, usually larger, organisation in an attempt to expand influence and expand their ideas and program. In situations where the organization being "entered" is hostile to entrism, the entrists may engage in a degree of subterfuge to hide the fact that they are an organisation in their own right.

It is a semi-official part of Trotskyism.

The Cultural Marxist concept of "The long march through the institutions" has similarities, but the "marchers" need not be part of a (concealed) organization, and the goal is not take control over a particular organization, but to influence general culture.