> who determines what is an emergency and if the expanded powers are needed?

Congress. But so far they're letting Trump declare emergencies left and right. All of his tariffs are being enacted under an "emergency" to bypass Congress (since under non-emergency situations, Congress sets tariffs).

Had the Democrats won control of the House/Senate, a lot of this nonsense wouldn't be happening (or even if conventional Republicans had control, which is why Trump 1.0, when conventional GOP held the senate, Trump couldn't go off the rails as he has now).