I'm unconvinced by the fracture line. The weakness of his case is shown by his examples: "...These conflicts have spread to all aspects of life, from sport to the brands of consumer products that red and blue Americans buy."
So...they wear different brands of trainers...amazing.
This "divide" is more apparent than real because political choice of Republican/ Democrat is and always has been strictly proscribed by the exceptionally small framing of the US Overton window. Foreign policy is essentially an ancient behemoth and the same under both parties: differing only in attempted speed of implementation whilst domestic policy is as ever capitalism red in tooth and claw.
Gore Vidal nailed the basics years ago, and it hasn't really changed:
“There is only one party in the United States, the Property Party … and it has two right wings: Republican and Democrat. Republicans are a bit stupider, more rigid, more doctrinaire in their laissez-faire capitalism than the Democrats, who are cuter, prettier, a bit more corrupt — until recently … and more willing than the Republicans to make small adjustments when the poor, the black, the anti-imperialists get out of hand. But, essentially, there is no difference between the two parties.”
It's where perhaps foreign and Domestic policy meet that there has been some movement: its about where those different brands of Republican and Democrat trainers are actually made.
Essentially the consequences of the love affair of US capital with Chinese state capitalism: the relocation of US manufacturing to China and Asia, the neglect of its own working population, the Rustbelt and the consequent dissatisfaction of the working people now increasingly without work and hope.
Add to this of course the ever increasing concentration of capital: Gore's "property" in fewer and fewer billionaire hands.
The billionaires funding both parties are making a killing from cheap offshore manufactured Asian imports and the increasing value of their rentier "property"... so the issue is really about how long this can continue without serious domestic disruption: in effect who is going to be best at pretending they will do something about it whist doing as little as possible to disrupt their paymasters from filling their boots while the going is still good.