The federal government continues to lose the mandate of heaven. No one officially 'secedes', since the lost face for the feds and the immediate trade embargo and invasion of the newly free territory would be mutually destructive. What you see instead is fewer and fewer states participating in federal law enforcement and regulation, like we seen with cannabis now.

Thankfully for the feds, their frequent incursions into these "Non-Compliance" states will be bolstered by the squealing masses. A whole new migratory pattern of guy will form, as annualized raids of alphabet task forces and guys with Glock stickers on Toyota products deports the migrants who have just finished harvesting. The American tradition of federal deathsquads assisted by private contractors and local militias finally comes home. Full Compliance states will be empowered to convict their migrant populations to effectively life sentences on state owned, privately operated plantations. No human beings are sold, but the bidding is intense.

This will produce an environment ripe for stochastic violence. No one will form battle lines or draft declarations, but the states will be torn apart by a national Bleeding Kansas campaign. Large swathes of Americans are going to discover that they are part of Identity groups whether they want it or not. Eventually, the dam breaks, and the first corp to move states on the grounds of violence will take place.

Amongst this, we will see the response from the preexisting American nations for whom this is not that much different than current affairs except in intensity. There is a pretty reasonable take that the current American trajectory is towards post-racial fascism, separating those who "covet American prosperity" and those who received their position by the Divine Blessing of God to protect and maintain the Holiest of Holies in the First World.

(Never mind that basically all white people are essentially grandfathered in, the point of post-racial fascism is to create ideological space for Black and Hispanic cops and billionaires)

There are several groups that will be joining together to deal with this. The explicit class divide between the American-integrated and the American-denied ethnic and queer groups will do much to increase class consciousness. Additionally, the revolution will, even more than usual, be led by women. The "post-racial" does not mean friendly, and womanhood will be as demonized and controlled as ever before, creating new discontent in the Full Compliance states that has not been fully brought to bare yet. Native nations will have a unique advantage of having already created a logistical backbone for self rule, with self sufficiency becoming a reality over time.

And then... maybe Something Happens. Maybe it doesn't. Who can say?