Actually the income tax is constitutional. It is simply misapplied. Which means that about 95% of Americans are not subject to (liable for) a revenue tax. If the government was run totally by the excise taxes that it was intended to run itself by, then you would have government spending within its means, because then the citizens could better control how government spent this money. The government would be downsized, the reigns to pull in out of control politicians would be easier to manage. There would be no policing the world. And imposing a certain brand of democracy on other nations.
With the present misapplication of the revenue laws, there is a mountain of money that the politician are all hungry to climb on top of. And they go mad when they get there.