'Raison d'être' - Huh?

'We've lost our  raison d'être ': Labour' Rachel Reeves says party is being deserted by traditional supporters.
They have been incredibly lucky to have got away with it for this long. The question I always ask to Labour voters is, "When did Labour stop representing ordinary, working people?" It usually stops them in their tracks and makes them try to answer the poser I have just given them. Labour is a party of the liberal elite - nothing more!
('Raison d'être' contains more than a clue!')