From the 1898 - 1987 (according to wikipedia) membership in any mayor union gave you an automatic membership in the Social Democratic party. Historically this was logical as the Soc.Dems were founded as a group of unions party wing. When they came to power they solidified their position as the unions party and marginalised those that would not play ball (the syndicalists unions).

The 1950'ies and 60'ies was probably the high point of soc dem control of the unions. With acceptance from the right wing, the soc dem governments (illegaly) put up a secret organisations to (illegaly) track communists, information then used to marginalise opposition within the unions. And to deny communists good jobs, and so on (the ordinary list of minor harassments).

In 1973 two journalists on a leftwing paper exposed the secret organisations, eventually leading to them being shut down (as far as we are told anyway). In this swedish version of the Watergate scandal the journalists were sent to jail (one year, for espionage) and the guilty politicians ruled on. Among them Palme who went on to become quite famous.

Eventually the coupling became untenable and in 1987 it was given up. The Soc.Dems membership shrunk quickly, still they remain the largest party.

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