In a new sign of increasing inequality in the U.S., the richest 1% of Americans in 2006 garnered the highest share of the nation's adjusted gross income for two decades, and possibly the highest since 1929, according to Internal Revenue Service data.

Meanwhile, the average tax rate of the wealthiest 1% fell to its lowest level in at least 18 years. The group's share of the tax burden has risen, though not as quickly as its share of income.

The figures are from the IRS's income-statistics division and were posted on the agency's Web site last week. The 2006 data are the most recent available.

The figures about the relative income and tax rates of the wealthiest Americans come as the presumptive presidential candidates are in a debate about taxes. Congress and the next president will have to decide whether to extend several Bush-era tax cuts, including the 2003 reduction in tax rates on capital gains and dividends. Experts said those tax cuts in particular are playing a major role in falling tax rates for the very wealthy.

Sen. John McCain has proposed extending the lower tax rates of 15% on long-term capital gains and dividends that apply to most taxpayers, while Sen. Barack Obama has said he will seek to raise them to at least 20%, the rate before the 2003 cut, and possibly higher.