By His Bootstraps by Robert A. Heinlein

A young man is pulled into the future through a mysterious temporal portal and becomes entangled in a chain of events where successive versions of himself recruit and manipulate his earlier selves; as political upheaval and personal ambition unfold, he discovers that his actions are both cause and effect in a closed time loop, forcing him to confront questions of identity, free will, and the paradoxes of self-consistency.