With no fanfare, President Bush yesterday signed into law the most sweeping corporate tax legislation in nearly two decades and the fifth major tax cut since he took office less than four years ago.

His signature means that Bush and the Republican Congress have fundamentally changed the U.S. tax code in ways large and small, for struggling individuals and corporate giants, bow-and-arrow makers and billionaires. Tax experts and academics remain deeply divided about the wisdom and effectiveness of these tax changes, but few disagree on their importance.

"Taken together, those policies and proposals represent a major shift in the structure, incentives, revenues and distributional effects of the American tax system," Brookings Institution economists William G. Gale and Peter R. Orszag wrote last week in the journal Tax Notes.

Since 2001, Bush has signed tax cuts of $1.35 trillion, $42 billion, $350 billion, $146 billion and $143 billion over 10 years. He has lowered income tax rates at every income level and carved out a new, 10-percent bracket from the 15-percent bracket that had been the lowest. The child tax credit was expanded from $500 to $1,000. Married couples, especially those in the middle-income range, received a significant tax break.

And the estate tax is scheduled to disappear in 2010. Already, Bush's cuts have raised the value of an estate exempt from taxation from $700,000 to $1 million. By 2006, an inheritance worth $2 million will be tax free.

But those tax cuts are only the most visible. The president has also raised the amount of child-care and dependent-care costs that can be written off a tax bill and has greatly expanded the amount of retirement savings that can be shielded from taxation. Taxpaying couples with incomes up to $160,000 can deduct thousands of dollars in tuition costs from their taxes until the end of 2005.

Businesses -- especially small businesses -- can deduct significantly more of their investments from their taxes. Investors have received a major cut in the tax rates on dividends and capital gains.

Until yesterday, large corporations complained that they had not been included in the tax-cutting bonanza. Not anymore: The new law showers businesses large and small with $143 billion in tax breaks over the next 10 years, with that cost offset by revenue raisers and loophole closures.

The $76.5 billion centerpiece of the new law effectively lowers the corporate income tax from 35 percent to 32 percent for domestic "producers" -- broadly defined to include old-line manufacturers, newspapers, home builders, even architectural and engineering firms. Also included is $42.6 billion of tax cuts for overseas profits.

Beyond those major provisions are hundreds of smaller measures that benefit restaurant owners and Hollywood producers; makers of bows, arrows and sonar fish finders; NASCAR track owners; and importers of Chinese ceiling fans. The law also includes a significant perk for some individuals, allowing taxpayers in states with no local income taxes to deduct sales taxes from their federal tax bill.

But all of this has come with a cost. All totaled, Bush has enacted more than $2 trillion in tax cuts since he took office. The budget deficit of $413 billion for the fiscal year that ended Sept. 30 was a record in dollar terms, surpassing the record of $377 billion set in 2003.

Even as the economy has recovered from the 2001 recession, taxes as a percentage of the economy have fallen for four straight years, to an estimated 16.2 percent of the gross domestic product in fiscal 2004. That is the lowest level since 1959.

To many conservative economists, such deficits are necessary to get the economy moving again. Edward C. Prescott, who won the Nobel prize for economics this year, commented after the prize was announced: "Tax rates were not cut enough."

A number of recent reassessments have given the tax cuts some credit for the recovery. Economists at Goldman Sachs wrote this week that households spent about two-thirds of the $38 billion in tax rebates mailed out in 2001, boosting growth in July, August and September of that year by 2.2 percent.

But other economists say the tax cuts could have been structured to have far greater economic benefits at lower long-term costs. They have also been fundamentally unfair, shifting more of the federal tax burden from the affluent to the middle class, critics say.

Although all income levels received a tax cut, a Congressional Budget Office study in August concluded that the share of total federal tax liabilities rose on the middle 20 percent of households from 10 percent, when the first tax cut was passed in 2001, to 10.5 percent this year. The share of the richest 20 percent fell from 65.3 percent to 63.5 percent.

In May 2003, President Bush signed a bill authorizing a $350 billion tax cut. Academics and tax experts are divided on the effectiveness of his five tax cuts.