Based on an exhaustive analysis of tax records and census data, the study reinforced the sense that while Mr. Bush’s tax cuts reduced rates for people at every income level, they offered the biggest benefits by far to people at the very top — especially the top 1 percent of income earners.

Though tax cuts for the rich were bigger than those for other groups, the wealthiest families paid a bigger share of total taxes. That is because their incomes have climbed far more rapidly, and the gap between rich and poor has widened in the last several years.

The study offers ammunition to supporters and opponents of Mr. Bush’s tax cuts, which are all but certain to touch off a battle between the president and the Democrats who just took control of Congress.

Democratic leaders have taken pains to avoid an immediate fight over the tax cuts, most of which are scheduled to expire at the end of 2010. But Democrats are looking for ways to increase revenue well before then, in part because they want to spend more on education and energy without increasing the deficit.

Economists and tax analysts have long known that the biggest dollar value of Mr. Bush’s tax cuts goes to people at the very top income levels. One reason is that two of his signature measures, tax cuts on investment income and a steady reduction of estate taxes, overwhelmingly benefit the wealthiest households.

But the Congressional study offers additional insight because it incorporates information about what people paid in 2004, the first year in which taxpayers could take full advantage of the cuts on stock dividends and capital gains.

The study estimates that the effective federal income tax rate, which excludes payroll taxes for Social Security and Medicare, declined modestly for people in the middle- and lower-income categories.

Families in the middle fifth of annual earnings, who had average incomes of $56,200 in 2004, saw their average effective tax rate edge down to 2.9 percent in 2004 from 5 percent in 2000. That translated to an average tax cut of $1,180 per household, but the tax rate actually increased slightly from 2003.

Tax cuts were much deeper, and affected far more money, for families in the highest income categories. Households in the top 1 percent of earnings, which had an average income of $1.25 million, saw their effective individual tax rates drop to 19.6 percent in 2004 from 24.2 percent in 2000. The rate cut was twice as deep as for middle-income families, and it translated to an average tax cut of almost $58,000.

In its report, the Congressional Budget Office estimated that the overall effective federal tax rate edged up to 20 percent in 2004, from 19.8 percent the year before.

But even with that increase, Americans faced lower tax rates than any time since 1979. If President Bush has his way, those rates could decline even more as the estate tax on inherited wealth is gradually phased out by the start of 2010.

Mr. Bush and his Republican allies in Congress want to permanently extend that tax cut and almost all of the others that Congress passed in his first term. The cost of doing that would be more than $1 trillion over the next decade, a cost that would hit the Treasury at the same time that the spending on old-age benefits for retiring baby boomers begins to soar.

The budget office offered little commentary on its new estimates, but many of its numbers spoke for themselves.