he joined the social democratic workers' party, or the sdap, in 1898 and broke with it in 1909 by sdap and was, with jan ceton, co-founder of the social democratic party, predecessor of the communist party of holland. wijnkoop was the leader of the communists in the years around world war i. he agitated fiercely against the social democrats and organized demonstrations in amsterdam at the amsterdam sdap-alderman floor wibaut. he left the cph in 1925, but returned to it later.