Oh yes they will. The justification given for progressive taxation has always been that a given percentage of their income hurts the poor more than it hurts the wealthy, so the wealthy should pay more. The left will say a flat tax is an assault on the poor.
They can say whatever they want, but every citizen needs to be vested in the country as a taxpayer.
There never will be a tax plan proposed by a conservative the liberals will not claim is an assault on the poor and a dozen other reasons they will come up with none of which would be true, it is built into the liberal agenda; the only good tax plan is a progressive liberal tax plan, that's just the way it has always been and always will be.