If wanted to talk about consolidating income, capital gains, and payroll taxes into one single tax, you'd say that's what you wanted to do.
Well my arbitrary 99.9% of all people consider those sources of income...that are not taxed equally to income generated by employment ie. Not flat.
No. You mean: Not equal. You're using the wrong word. The word "flat" in the phrase "flat tax" refers to the graph of rate versus volume. There is no equivalent concept of "flat" when comparing tax rates on payroll or tax rates on long term investment to tax rates on direct compensation. They really are completely different things. I get that some people use the phrase "flat tax" as a talisman to mean whatever ideal and fair tax system they'd like to implement, but that's more a matter of people misusing the phrase than any rational implementation of its actual meaning.
Look though. Having said all this, if we want to discuss tossing all our different taxes into one bucket, we can discuss that. However, I also don't think that most liberals really want that. The outcomes would be almost completely opposite of the kinds of tax policies that liberals push for. I just find it incredibly amusing because despite liberals constantly insisting that our current tax system is unfair, that system itself is largely the result of their own political agenda and desires. The very things that make our taxes "non-flat" (in whatever context you want to use), are things that overwhelmingly benefit the social agenda of the left over the last century. When liberals complain about our tax system, it's not because it's not flat, but because it's not "unflat" enough for them. They want more of the tax burden borne by the rich and more of the benefits of those taxes spent on the poor. Any argument for any sort of flat tax system would go in the opposite direction.
Payroll taxes are already "flat" (to a point). What liberals want isn't to make other income that poor people pay flat, but to make payroll taxes more like regular income (ie: progressive). That way the poor can reap the benefits of social security and medicare, without having to pay for it (or pay less for it). Liberals love our progressive tax rates on regular income. This is why I keep wondering why any liberal would argue for a "flat tax". That's not really what you want, no matter what language you use. You don't want all earnings consolidated into one lump and then taxed at the same flat rate regardless of volume and without any deductions. You want all earnings consolidated into one lump and then taxed progressively with as many deductions and credits for those in the lower income brackets as possible.
You can call that a "flat tax" but at that point the label ceases to have any meaning.