Here is the text of the Sixteenth Amendment to the U.S. Constitution:
Amendment 16 - (Passed by Congress July 2, 1909. Ratified February 3, 1913) - The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration.
It should not take a "constitutional scholar" to see that the new federal tax law is NOT unconstitutional.
And to think, it was "progressives" who pushed for that stupid amendment in the first place...
I'm sure that their claim, whatever it might be, won't be based on an infringement of the 16th. Most likely it will be an equal protection claim. That would still be very dubious since the rules apply equally to residents of all states, and it's only the behavior of the states themselves that lead to different results.