A flat (income) tax is unjust to most competitors, i.e., to most citizens. It promotes wealth and begets wealth, rather than hard work begets wealth. Without some negative feedback, we end up with Monopoly the board game, where as you get wealthier, your leverage increases. making it easier to get even more wealthy -- which ends the game, but which would destroy the fun of it if every new game (every new year) everyone had to start in the place they left off -- as people are forced to do in real life.
A failure to curtail wealth that begets wealth over time leads to relatively few very powerful proprietors that have huge advantages over the lives of most people because of that vast leverage their growing wealth gives: a feed-forward system, where all else being about equal, the more you have the easier and with lower risk you can make more, leading to even more leverage, etc.
The US lacks a wealth tax (at the federal level). A progressive income tax is a substitute for that, but it can't replace a wealth tax. Wealth is private sector leverage, thanks to the government Uncle Sam. Do those with more wealth pay higher taxes on that private sector leverage and power the government makes possible? This existing unfair system would contrast with a scenario where everyone believes they should have equal access to land and to many other forms of wealth at some point in their lives, for example, an initial split of US currency as you might see in Monopoly at the beginning of the game -- the only time that game is fair -- or otherwise, that they own an equal part share in all resources at least until another person negotiates directly with them.
Can we imagine how costly it would be for Romney to hold significant amounts of real estate against the other 300 million negotiating citizens? This service that gives some exclusive control over vast resources that were either natural in existence or were created by others (workers or factories themselves created by workers or by robots that were created by workers, etc) is something that fairness dictates should be taxed. This privilege and opportunity costs to every other citizen does not happen for free; why should the majority of workers with little leverage and hence undervalued wages subsidize this power that we give to the privileged (mega) proprietors aggregating, re-leveraging, and aggregating some more?
With a wealth tax, idle resources would be sold or put into use, moving resources into more efficient uses and into production. This would fight unemployment. This would give ordinary people (especially new and small entrepreneurs) extra leverage that the government otherwise takes from them and gives to the very wealthy. With this leverage incomes would rise for those doing the majority of the work. With this leverage, a person would be allowed to feed themselves rather than depend on the blessings of land owners.
Hard work should beget wealth. Isn't the the American Way, rather than wealth begets wealth? Shouldn't our rules charge proper fees to leverage and power (e.g., to wealth backed by the US government, a stable US dollar and society, streamlined title ownership structure, etc)?
Let me ask explicitly: Is it a fair playing field in determining wages if one group year after year starts off with a very strong hand? Is that balanced gaming? Is that fair? Would you bother to play Monopoly if one player (the "top 20 percent") always started off owning 85 percent of the properties and 85 percent of the bank money?
Is it fair to young kids who, rather than have $1 million earning interest for them from birth and lots of advice and opportunities to make mistakes and recover, instead have little good business advice or just plain bad advice and weak role models, weak access and few resources and opportunities to make mistakes and learn? Does this imbalance from birth or from year to year lead to anything but very unfair competition and the consequent movement of wealth into fewer hands?
Why don't we have a wealth tax? Why does the government "borrow" the US debt from already wealthy folks and maintain title to property that accumulates across generations in few hands instead of giving every new baby citizen a fair playing field and creating money in a controlled fashion without going into debt?
And why does the US give up so much land to private ownership instead of leasing -- a practice in the private sector which is a more lucrative proposition that would give citizen-shareholders a better return on their citizenship and natural rights in equal parts to resources?
Do we really want a few people pulling the strings and not paying a fee for that vast power from our public Uncle Sam? Is that fair that we can't contest title rights (and native Americans got run over, remember) or get an even break when we are born? Why would I as a voter support a playing field that is lopsided in favor of whoever happens to be very wealthy at a given point in time?
A progressive income tax system will hopefully be improved to include a wealth tax. Those empowered by our government should pay a fee for that service and as compensation to all the rest of us losing access and leverage.
A fair playing field would lead to a lot more proprietors and to a path to making money that would allow common workers and small business owners a fair shot against the largest proprietors. Until we fix the playing field, we will continue to have many who work little and make gazillions while most do the vast majority of the work, under-paid (since Uncle Sam took away most leverage), and yielding their contribution to profits to the proprietor with the upper hand.
A fair playing field means more job creators, more upward mobility, more disposable income spread across the population. This is closer to a fair free market if we accept that the public military, courts, and infrastructure do add value to those controlling wealth. The government is our contract with each other. Shouldn't our contract stipulate that fair fees be charged for anyone endowed with particularly large power against the rest of us?
We can easily avoid most socialism, but we do want fair fees so that wages reflect hard work and skill in some proportionally manner rather than be so skewed towards those pulling strings thanks to their huge supply-demand advantages over most wealth and resources needed. We should give all babies a fair playing field, for none did anything greater than any other before they were born. We should not subsidize the very wealthy families who then pass on that subsidy to a very few (their kids, for example). Even in a broken game like Monopoly, at least for one second at the beginning of each game, the playing field is level. We should and can do better in real life.
So when people press for flat income taxes, remind them that we need to go in the other direction and actually start charging fees for the power given by our government, on behalf of the rest of us and subsidized by us, to those who have the most wealth already.