A more thorough explanation:

Definition: The governmental-function theory is a principle in constitutional law that characterizes private conduct as state action, especially for due-process and equal-protection purposes, when a private party is exercising a public function. This theory means that private entities performing public functions are subject to constitutional constraints.

Example: A political party, which is a private entity, cannot exclude voters from primary elections based on race. This is because the political party is performing a public function, and therefore, its actions are subject to constitutional constraints.

Explanation: The example illustrates the governmental-function theory because the political party is performing a public function, which is conducting primary elections. Even though the political party is a private entity, its actions are subject to constitutional constraints because it is performing a public function. Therefore, the political party cannot exclude voters based on race, which would violate the equal-protection clause of the Constitution.