John Adams recognised the danger that laid down the road in 1780. The “greatest political evil” to be feared under a democratic constitution, the US founding father warned, was the emergence of “two great parties, each arranged under its leader, and concerting measures in opposition to each other.”

The raw tribalism that's emerged as the defining force in 21st century US politics would have confirmed Adams's gloomiest fears. What we have seen is the rise of identity partisanship.