The tax rate declined across all income levels — but more so in the top brackets, the report said.
The study found that the effective tax rate for the top 1 percent of taxpayers dropped from 33 percent in 2001 to 26.7 percent this year, a decline of 19 percent. The middle 20 percent of taxpayers saw a decline of 4 percent.
The study, requested by congressional Democrats in May, quickly provided fodder for the presidential campaign over the fairness of more than $1 trillion in tax cuts Mr. Bush has pushed through Congress since taking office.
"Over the last four years, the burden of taxes has shifted from the wealthy to the middle class," Democratic presidential nominee John Kerry said at a campaign event in Springfield, Oregon. "The middle class is paying more taxes."
The White House defended Mr. Bush's tax cuts, saying those in lower- and middle-income brackets also benefited.
People in the top 20 percent of incomes, averaging $182,700 a year, saw their share of federal taxes decline from 65.3 percent of total payments in 2001 to 63.5 percent this year, according to the study by congressional budget analysts.
In contrast, middle-class taxpayers — with incomes ranging from $51,500 to $75,600 — bear a greater tax burden. Those making an average of $75,600 had the biggest jump in their share of taxes, from 18.5 percent of all payments in 2001 to 19.5 percent this year.
"George W. Bush keeps trying to mislead Americans into thinking we're turning the corner, but truth is that he is turning his back on middle-class families," Kerry spokesman Phil Singer said. "The Bush policies are exacerbating the squeeze that working families have been feeling for the last four years."
Bush-Cheney campaign spokesman Steve Schmidt said, "Because of President Bush's policies every American pays less in taxes today than they did before he became president...John Kerry has promised to raise taxes during the campaign. That is the clear choice Americans will have in the fall elections."
The study is based on figures in 2001 and assumes no changes in wealth distribution from increases in income, dividends or capital gains.