In a lunchtime speech to the Chamber of Commerce in Concord, N.H., McCain suggested it would be irresponsible to use most of projected budget surpluses to cut taxes -- as Republican presidential rival George W. Bush would do -- without planning for the retirement of the big baby boom generation, which will push Social Security toward insolvency.

"Others suggest that we should use every penny for tax cuts, forgetting we have promises to keep and a fleeting opportunity to keep our word without imperiling the economic future of our children and generations to come," McCain said.

The Arizona senator said his plan would provide a significant tax cut while setting aside 62 percent of the budget surplus that is not already earmarked for Social Security to help keep the retirement program afloat. The proposal would also set aside 10 percent of the non-Social Security surplus for Medicare and 5 percent to reduce the national debt.

To help pay for the changes, McCain said he would raise some $150 billion by closing a long list of corporate tax loopholes and shelters.

"Every tax dollar now wasted on special breaks for oil companies, ethanol giants, insurance companies and the multitude of other special interests with their armies of lobbyists, is now at risk," he said.

The Republican presidential front-runner, Texas Gov. Bush, is pushing a five-year, $483 billion tax relief plan that, among other things, would gradually cut all income tax rates, abolish estate taxes and double the $500 per-child tax credit. Critics say it would leave precious little for Social Security or Medicare.

In a statement today, Bush said he and McCain "have a fundamental disagreement" over taxes and how to use projected surpluses.

"I believe surpluses should be passed back to hardworking Americans," Bush said. "Senator McCain's plan would leave more money in Washington to be spent on more government."

Much of McCain's plan has been made public before: Like Bush, the Arizona senator would increase the $500-a-child tax credit to $1,000. But McCain also wants to expand the bottom 15 percent tax bracket to couples with taxable income of $70,000, up from $43,050 now, and to $35,000 for singles, up from $25,750.

McCain would also create new "family security accounts" in which an individual in the new 15 percent income tax bracket could save up to $3,000 tax-free per year, $6,000 for married couples, aides said Monday on condition of anonymity.

Like a traditional individual retirement account, the money would be taxed as regular income when it is withdrawn. Unlike a traditional IRA, the money could be withdrawn after only one year without penalty and could be used for any purpose.

The idea is to enhance family savings with as few restrictions as possible, a McCain aide said.

Bush spokesman Ari Fleischer, however, said the savings proposal is "small dollar" compared to the amount middle-income people would save under Bush's broader income tax cuts. McCain's expansion of the bottom tax bracket, Fleischer said, would do nothing for the 71 percent of taxpayers who are already in it.

If elected, aides said, McCain would continue pushing up the income limits on the 15 percent income tax bracket in an effort to one day achieve a single, "flat" tax advocated by many GOP fiscal conservatives and championed by another GOP presidential challenger, publisher Steve Forbes.

McCain is working at establishing a flat tax "from the bottom up instead of the top down," the McCain adviser said.

Another major new item in McCain's proposal, which he will flesh out further today, would raise $150 billion over five years by closing corporate tax shelters and other breaks. In the Senate, McCain is a frequent critic of such arrangements.

In addition to setting aside billions to keep Social Security solvent, McCain wants to allow workers to save up to 20 percent of the payroll taxes they and their employers pay into the system for private investment accounts. Details of that plan would come later, campaign aides said.

McCain also proposes easing the "marriage penalty" paid by millions of two-income couples by raising the standard tax deduction from $7,200 to $8,600.

His plan includes an unspecified tax deduction for long-term care expenses, expansion of tax-free accounts for medical costs and education and elimination of income taxes for military personnel deployed or stationed overseas.