The Social Democratic Party of Germany was founded in 1863 but would not win an election until 1912. What is the earliest possible POD for a SPD victory?
they actually won the popular vote in every election from 1890 onwards, but thanks to the German electoral boundaries they never won that many seats. So if you wanted an earlier SPD victory you might want a POD where Frederick III lives longer, since he was liberal enough to want to make both the parliament and its electoral system fairer.
Frederick III lives longer, reforms political system
perfectly possible that there could be an SPD government by 1900, if not before, thereby beating Australia to have the first Socialist government in the world.