The flat tax is a gift to the donor class. Never support a candidate who proposes changes in the tax code without reductions in spending first. If you have no political power any change made through government will be a disadvantage to you.
There is one good reason for taxing the rich out of proportion to the rest of society. The rich lobby constantly to increase the size and role of government. The rich reap enormous benefits from the growth of government and thus should pay the majority of its costs.
The most valuable attribute of any tax is in keeping citizens aware of the cost of government. When spending is increased, citizens need to experience a loss of disposable income. Changes to the tax code in tax code by proposed politicians always facilitate more spending and guarantee citizens won’t object. If they don’t feel it, they won’t care.