So many people have supported a national flat sales tax. In the global currency reset circles, a flat tax of 17% or 14% had been predicted to start January 1st, 2020. But the flat tax is no more Constitutional than the graduated income tax. The Constitution for the United States of America forbids any direct tax from the federal government within the states without regard to apportionment.
As we see in Article I, Section 2, Clause 3