This includes

Any dramatic change to the governing apparatus at the federal level: extending presidential terms, prosecution of political enemies in some dramatic form, changing how elections are held, a dramatic change in state power

A crisis between different branches of the government: I.e the courts rule that a certain candidate won a major election, the house disagrees and tries to fight it leading to a crisis. Or a certain body declares another one illegitimate

Any sort of major change is civil liberties, any major act of insurrection (that is successful), a successful independence resolution by a state…etc

Basically anything that profoundly changes how the US operates and has wide reaching impacts in people’s lives, will be highly polarizing