Tax protestors love to quote Article 1, section 2 of the U.S. Constitution, which provides in part:

The income tax, these protestors say, is a "direct" tax, and therefore, by virtue of Article I, section 2, it must be "apportioned" among the states so that the income tax collected in each state would be proportional to each state's population. Of course that's not how income tax works (you just pay according to your income, without regard to how many people live in your state), so these protestors claim that the income tax is unconstitutional.

The question of whether the income tax is a "direct" tax, as that term is used in the Constitution, has a long and rather complex answer. But the short and easy answer to this protestor argument is that it doesn't matter whether the income tax is "direct" or not, because the apportionment requirement of Article I, section 2 no longer applies to income taxes. Yes, it's true, under the original Constitution, direct taxes (whatever those are) had to be apportioned, but the Constitution has been amended, particularly by the Sixteenth Amendment, which provides:

Clear and simple. Whatever "direct" taxes might be, and whatever the rule is for other kinds of direct taxes, Congress can now impose an income tax without apportionment among the states. The rule of apportionment does not apply to income taxes by virtue of the Sixteenth Amendment.

The end.

But wait! Protestors point to Supreme Court opinions stating that the Sixteenth Amendment "conferred no new power of taxation." For the response to this hopeless follow-up to a hopeless argument, see here.

And they also say the Sixteenth Amendment was never ratified! Sheesh. See here.

And if you really want to know whether income taxes are "direct" taxes (even though it doesn't matter), see here.