The American Democrats and Republicans are often labelled with the colours blue and red respectively. However, the historical "blue states" and "red states" were once inverted from their current meanings.
In the 1860 presidential elections (in which Abraham Lincoln was elected US president), it can clearly be seen that Lincoln's Republican Party won the majority of the northern states, while Douglas and Breckinridge (Democrats) were mainly supported by the Southern states.
In 1916, when Woodrow Wilson (a Democrat) was elected for his second term, support still came mostly from the South, while the majority of the Northern states voted for the Republican Charles Hughes.
However, the traditional Red-Blue state split became less obvious in the middle 1900s, with elections during this period showing less obvious north-south splits. At the end of the century, in the 1992 elections, the contemporary pattern of North-Democrat and South-Republican became obvious in the Clinton vs Bush elections:
The most recent election (2012) of Barack Obama (Democrat) vs Mitt Romney (Republican) showed an almost 100% flip from the 1860 states map, with Obama winning almost all of the Northern states, and Romney winning almost all of the Southern states.
When did this switchover of party allegiance between states occur, and why did it happen?