Usage patterns
〈人〉が政界に入るfor someone to enter the political world (become a politician)
He quit his company and entered the political world.
〈人〉が政界から引退するfor someone to retire from politics
She retired from the political world, citing her age.
〈人〉が政界に進出するfor someone to move into politics (from another field)
A famous actor moved into the political world.
政界の重鎮a heavyweight/senior figure in the political world
He has long been known as a heavyweight in the political world.
