The 16th amendment to the US Constitution permits a graduated or progressive income tax at the Federal level. It was adopted in February, 1913 and has been opposed by the GOP ever since. Until they can engineer its repeal, the Republicans have been working to make the law ineffective. Their definition of a progressive income tax is an income tax on the income of progressives only. Their version of the tax is one that most people would define as regressive, the more you earn, the less tax you pay. For those earning the really big bucks, the tax rate is zero, or in some cases, less than zero. Personally, I believe that the progressive income tax is the fairest tax of them all.
P. S. The 16th amendment will celebrate its 100th birthday next year in February.