Some Democratic presidential candidates have floated the idea of a new U.S. wealth tax (one version: 2% on net worth > $50M; 3% on net worth > $1B). There are a couple of ways that this tax could be implemented in a future version of everyone's favorite document: IRS Form 1040.
That's illegal since it violates the US Constitution, specifically Article I Section 9 Clause 4:
No capitation, or other direct, tax shall be laid, unless in proportion to the census or enumeration herein before directed to be taken.
That's why you have a 16th Amendment.
Let's say you wanted to levy a federal income tax of 15% on anyone earning more than $20,000/year.
That's illegal under the Constitution. The only way to make it legal under the Constitution is to apportion the tax by population.
So, people living in California would pay 30%, Texas 28%, Florida 27%, New York 26.5%, Ohio 22%...Kentucky 15%...Vermont 2.5%, Wyoming 1% like that.
The 16th Amendment lets you get around that and levy one single uniform tax for everyone:
Amendment XVI
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.
See the key clause? "...without apportionment among the several states..."
That's what that means.
But, note the other key clause: "...on incomes, from whatever source derived..."
You can only tax income, not wealth.
You can tax income derived from labor, rentals, sales or anything else, but you cannot directly tax assets or wealth.
If you want to tax assets or wealth, you must apportion it by population, which means this:
People living in California would pay 30%, Texas 28%, Florida 27%, New York 26.5%, Ohio 22%...Kentucky 15%...Vermont 2.5%, Wyoming 1% like that.
Can you not see a stampede of people fleeing the five most populous States -- California, Texas, New York, Florida and Pennsylvania -- for lower populated States?
Because that's what would happen.
Or....you would see wealth transfers to lower populated States.
If I lived in California, I would open a bank account in North Dakota, transfer my wealth from California to North Dakota and close all of my California bank accounts.
If I owned any business in California, I would immediately terminate the registrations, domicile the businesses in North Dakota or Montana or Vermont, then re-register the businesses in California as foreign entities.
I would buy a house in one of the least populated States and transfer all my assets there.
I would do that in order to avoid the tax.
A wealth tax is illegal and will never happen unless there's an amendment to the Constitution like this:
Amendment NN
The Congress shall have power to lay and collect taxes on assets without apportionment among the several states, and without regard to any census or enumeration.