I guess that is the congregation point, big cities everywhere.
Looking at the population stats for all of the largest cities in America, only about 5 of the top 90 are red... they're basically anomalies. It is when you get down to around #93 that you get about a 50-50 mix... and that's where the cities are approximately 240,000 in total population... so my hypothesis is that this level is where humans as a species being to change their behaviors and their norms and their psychology, from freedom and independence and self-sufficiency into dependence and security and more Marxist ideals.
Looking back in our history, it wasn't until 1840 that NYC became the first US city to surpass 300,000.
Philly exploded from 121k in 1850 to 565k in 1860.
Brooklyn was the 3rd city to pass 250k in 1860 as well, but that's basically an extension of NYC.
It wasn't until 1890 that we had even just 10 cities over 250k.
NYC doubled from 1.5m in 1890 to 3.0m in 1900.