Bitsy, we have that problem right now with our current tax code. These loons can raise/lower capital gains, personal marginal rates, corporate taxes....etc., at will by legislation with an agreeable President.
I have been saying let's do the flat tax but with significant congressional oversight that would put a lot of stopbap measures in place to prevent easy access/change to the rates. Super majorities, in other words.
If I had complete faith, or even a little faith, in our government, I would be all for a simpler,fairer flat tax. You mention an oversight - a super majority - correct me if I am wrong, but didn't that majority bring us obamacare? If we were dealing with statesmen, I wouldn't be as apprehensive, but we are dealing with a bunch of career politicians and a democratic party that has been hijacked by leftists.