Texas Gov. George W. Bush will propose a $483 billion tax-cut plan today that would focus its deepest reductions on the working poor and middle class and become the centerpiece of the Republican front-runner's economic plan.

While the five-year Bush tax plan would benefit wealthier Americans, roughly half of the overall relief would be targeted to middle- and lower-income families, according to campaign aides. That emphasis on the poor would mark a clear departure from more traditional conservative GOP tax policy and would leave Bush backing a tax package far more generous than the one proposed by congressional Republicans earlier this year.

Bush has worked diligently to portray a kind of big-tent conservatism that rejects stereotypes of a Republican Party that is insensitive to the concerns of the poor. Advisers to Bush who outlined the plan yesterday said the governor's philosophy was that "America should not be putting up toll gates to the middle class."

They also believe that Bush's approach will have strong appeal to Democrats. The proposal includes relief from the taxes some married couples pay, a gradual elimination of the estate tax and added tax incentives for charitable contributions.

But in crafting his new approach, Bush has proposed a much bigger tax cut than even the GOP tax legislation that was vetoed by President Clinton in September. That plan would have cost $155 billion over five years or $792 billion over 10 years. Bush aides said his plan would cost $132 billion a year for the sixth through 10th years, or $1.14 trillion over a decade. Bush also is relying on rosier long-term projections for the amount of revenue the government will take in than have been forecast by the Congressional Budget Office (CBO).

Bush, like the congressional Republicans, would pay for his tax cuts with budget surpluses projected for the coming years, excluding those generated by the Social Security payroll tax. But those surpluses will total only $463 billion from 2002 to 2006, according to the CBO, short of what would be needed to cover the tax cuts and any new domestic programs a Bush administration might propose.

In drafting his economic plan, Bush and his advisers have had to wrestle with a new budgetary reality on Capitol Hill. Congressional GOP leaders and Clinton have entered into an informal pact not to tap the current Social Security surpluses for either spending or tax cuts.

In the past, both parties routinely used the Social Security surplus to finance the day-to-day operation of the government. But with both parties eager to demonstrate their concern for the future of Social Security as the baby boomers begin to retire in the next decade, a Bush administration would be hard-pressed to advance a program of deep tax cuts and increased spending for defense and education without dipping into Social Security funds.

Bush advisers, however, believe that the CBO forecasts have been far too conservative and that the government will take in much more than it needs to operate. The Bush camp is assuming that the economy will grow in the coming years at an annual rate of 2.7 percent, adjusted for inflation, instead of the 2.3 percent forecast by CBO. The U.S. economy grew at a hefty 5.5 percent annual rate in the last quarter.

"The estimates that underlie our assumptions are very conservative," Bush spokesman Ari Fleischer said yesterday. "We believe, just like the Blue Chip forecasters' estimate, that 2.7 percent is a very reasonable, realistic projection."

In a highly unusual move, the Bush campaign released its economic platform a day early to select news publications on the condition that they not seek reaction from independent analysts or rival campaigns. The campaign said it was trying to prevent Bush from being preempted by his rivals before his lunchtime speech presenting the plan today at the Des Moines Chamber of Commerce. Late in the day, the campaign agreed to allow reporters who accepted the arrangement to discuss broad outlines of the plan with economic forecasters.

Asked about the plan, Bill Gale, senior fellow at the Brookings Institute specializing in tax policy, said that he saw nothing that represented "a dramatic shift" in the current tax structure. "He's not endorsing the flat tax, for instance. He's not saying, let's throw out the system."

Bush campaign officials said yesterday the plan's emphasis on the working poor, what they refer to as the "social mobility" plank of the tax platform, was meant to remove some of the economic barriers that prevent people from attaining the middle-class American dream. When Bush endorsed the GOP tax plan last spring, he said that he would have preferred more benefits for the working poor but offered no specific proposals.

Under the plan he will present today, the tax rates in all five income tax brackets would be reduced, with the lowest rates getting the largest percentage reduction: The 15 percent tax rate, for couples making up to $43,050, would be reduced to 10 percent, a 33 percent reduction.

Couples in the 28 percent and 31 percent tax brackets, which includes those with incomes up to $158,000, would see their tax rate cut to 25 percent. The top bracket of 39.6 percent would be scaled back to 31 percent, a 22 percent reduction. Because the wealthiest Americans pay the most in taxes, they inevitably will realize the largest net savings from any across-the-board tax cut.

Bush's plan would provide additional assistance to low-income families by exempting a portion of their income from any income tax and by doubling the $500-per-child tax credit. About 6 million working poor families with children who now pay income tax would be dropped from the tax rolls, according to Bush aides.

The Bush plan would also provide substantial tax relief to married couples, an idea popular in both parties. To do that, Bush would resurrect a tax provision that was phased out in 1986 that would allow taxpayers to deduct 10 percent of the earnings of the spouse with the lower income.

Bush's economic team is led by Lawrence Lindsey, a former Federal Reserve governor, who also advised former presidents Ronald Reagan and George Bush. Much of the rest of the team of economic advisers includes academics from Harvard, Stanford and Columbia universities.

One of those advisers said the younger Bush issued a mandate to the group when it began meeting earlier this year: "What Bush told us to do as advisers was identify the biggest problems in the tax codes, the biggest disincentives and reduce them and that's what we did."

But Bush is certain to come under attack today from his opponents on the right and left. GOP rivals Steve Forbes and Gary Bauer, for instance, have proposed versions of the flat tax, a more radical idea that appeals to many conservatives. Vice President Gore and Bill Bradley, who have proposed spending much of the non-Social Security surplus on domestic programs, are likely to take a dim view of a plan that drains most, if not all, of that surplus.