While it's true that fair is hard to define objectively progressive tax rates are easy to support from quite a few underlying premises: 1) utility maximization

While it's true that fair is hard to define objectively progressive tax rates are easy to support from quite a few underlying premises: 1) utility maximization 2) Rawlsian justice 3) fee for service model based on benefits received from government It's actually pretty hard to justify a flat tax from most underlying axioms. I think you'd actually have an easier time justifying a flat fee where everyone pays the same $ value regardless of income. I also contest your assertion that "Conservatives think it's 'fair' that everyone pays the same rate." While that might be true for some conservatives, political support for progressive tax rates go much beyond Democrats.