I think that explains the immediate effect. You can of course then wonder why the population is distributed that way, but I feel I've done enough here :)
>Yeah, there is a pretty distinct line from Winnipeg to San Antonio, separating light from dark.
That's the I-35 corridor. An interstate highway running north/south all the way from Mexico to Canada. It was probably the most hyped/utilized part of NAFTA. I live in Dallas where I-35 runs through (actually splits in Waco to cover both Dallas and Ft. Worth, reconnecting north around Denton). I grew up in a small city south of Dallas, and remember all of the highway construction that got spurred by NAFTA. That area (Ellis county) also had a construction boon due to the Super Conduction Super Collider.
If you look at a population density map, it looks quite similar: http://dafi1637.blogspot.com/2017/10/united-states-populatio...
I think that explains the immediate effect. You can of course then wonder why the population is distributed that way, but I feel I've done enough here :)