Legal or not, the sad fact is that the IRS isn't even needed, if people could change their philosophy of government. A country is a lot better if the people keep more of their money and give less to an incompetent, wasteful, and generally stupid government that largely wastes it.
I like the flat tax plan, though I think Huckabee's 23% is ridiculously high. I REALLY like Ron Paul's idea to get rid of the IRS and stop all this BS foreign and domestic spending that is the "need" behind the revenue it brings.
With that said - I am one of the people who believes that the income tax IS illegal, and though I do pay taxes, rest assured that I have used every single loophole and legal practice available to minimize my liability. I'm repulsed by what the govt does with my money, so I don't want to give them .01 more than I can. 
There is no Constitutional basis for a lot of things the govt does - like the "war on drugs", welfare, or, say, the entirety of the Bush Administration. But they do it anyway.
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Google isn't as dumb as you think.