It has been proposed to increase revenue by reducing or eliminating loopholes. Which loopholes are the problem. If it's just on luxury goods, one set of citizens are more sharply affected and view the change as unfair. If it's on charitable donations, a different set of citizens are more sharply affected and view the change as unfair.
With the hundreds of deductions on the books, debate may rage for months over which and how to change these tax codes. Nothing will get done. The economy is not helped.
A limit based solely on liability bypasses these arguments.
You may use any and every deduction currently allowed by the tax code, in any combination you wish, to the limit of 50% of your liability.
Tax rates are unaffected.