Social democracy … rise and decline
The movement started in 1889 with the founding of the German Social Democratic Party. Its popularity and influence led to the establishment of similar parties in most of the advanced democracies by 1900.
This graphic happens to show vote percentages for the social democratic party of Sweden, specifically, but it’s pretty representative of most other European moderate-left parties. Notice strong growth 1900–1950, then some decline (not precipitous in most cases, except in France) over the last forty years: