The burden first is going to be on the challengers. They will have to establish that they have standing to sue and that there is a Constitutional basis for their positions. They may not get past those hurdles. The Feds don't have to provide a justification until they do, and they will fight like demons to put every procedural hurdle in the way of the challengers.
Well, yes, I’m assuming standing and statement of a cognizable claim, etc. Not saying those aren’t big mountains to climb. But my real point is what is the basis of the feds’ substantive response? It’s just not very clear where they would go with this, assuming they want to avoid arguing that it - wink! - really is a tax.