That would be an incredibly risky escalation, and it would be a stupid ultimatum to issue.

The people, or even states, could escalate in response. The worst case is escalating to violence; ICE isn’t trained, equipped, or numerous to deal with deploying into a violently hostile area. The army could, but then we’re in full blown civil war.

A more realistic middle ground is that it pushes people or states into nonviolent non-compliance by eg refusing to pay federal taxes. Frankly if California and New York alone stopped paying federal taxes the system would probably crumble.

ICE isn't trained for anything, they're just proud boys in tacticool gear. But the point of them is not to be effective, it's to cause headlines.

That's not how the political reality of exacting mostly voluntary compliance from the masses works.

Yeah because that works out really well in history!