The Joint Economic Committee released a report on our federal income tax system. Check out the chart at TaxProf Blog that shows the distribution of taxes paid. The bottom 50% in adjusted gross income pay only 3.46% of the total income taxes paid, which leaves 96.54% paid by the upper 50% of adjusted gross income. What's more is that the top 5% in AGI pay over 1/2 of the federal income taxes, and the top 1% pay over 1/3.
It seems to me that a fair and efficient tax system cannot be a tax system with 1/2 the population paying essentially nothing for the goods and services provided by the national government.