According to the Treasury Department, by way of the IRS, the top 0.1% of US earners make 9.1% of the country's total income. However, they pay 17.4% of the total taxes. The top 1% earn 19% of the total income, but pay 36.9% of the total tax. In contrast, the bottom 50% earn 13.4% of the income and pay only 3.3% of the total tax.
To highlight the "fairness" of the US tax structure, let's compare a US household consisting of a married couple with two kids, making the average wage, with the same household in other countries. In the US, this family would pay 11.9% in taxes. In Iceland and Ireland this family would pay 11% and 8.1%, respectively. If you're Canadian, you pay 21.5%. Move to Japan and you pay 24.9%. England, Germany, and France? Try 27.1%, 35.7%, and 41.7%. So, according to this data, by the world's standards the US middle class is not overtaxed.
Last July, the NY Times ran a piece which showcases the fact that "tax revenues are increasingly dependent on the fortunes of the very rich." The Times also points out that "the top 10% of taxpayers- those with incomes above $100,000- provide about two thirds of income tax revenue."
When looked at in these terms, "tax cuts for the rich" doesn't have the same sting as a campaign slogan. If anything, the NY Times' data suggests the rich are shouldering too much of the nation's tax burden, making it extremely difficult to create accurate budgets. In effect, the US government is beholden to the success or failure of the stock market. Therefore, because of the progressive nature of the US income tax system, Congress is faced with the same budgeting difficulties that any other commission based household has.