WASHINGTON — In attempting to fix Social Security's long-term problems without raising taxes, President Bush has chosen to recast the 70-year-old retirement program as one that would keep the lowest-income workers out of poverty but become increasingly irrelevant to the middle class and the affluent.

Under Bush's approach of "progressive indexation," a typical low-income worker who earns about $16,000 a year today would be entitled to retirement benefits equal to about 49 percent of his or her wages, the same amount promised today.

But those earning an average income, about $36,500 in today's dollars, would see big changes. Instead of replacing 36 percent of that person's working pay, as promised under today's system, benefits would cover 26 percent of that person's pay by 2075. And people who earn $90,000 a year in today's dollars, who would continue to pay as much as ever in taxes, would receive benefits equal to 12 percent of their pay.

In his radio address yesterday, Bush sought to cast himself in the Democrats' traditional role as a defender of the poor. "Benefits for low-income workers should grow faster than benefits for people who are well off," he said. "By providing more-generous benefits for low-income retirees, we'll make good on this commitment: If you work hard and pay into Social Security your entire life, you will not retire into poverty."

But critics, including most Democratic lawmakers, said such an approach would undermine a central bargain conceived during the New Deal: Social Security is not only a welfare program for the poor but a form of social insurance that people of all incomes pay into and reap rewards from.

"Social Security is not a poverty program, it is a retirement system people have worked hard for, paid into and have earned," said Rep. Sander Levin, D-Mich.

If it becomes increasingly irrelevant for middle-income people, critics said, Social Security will become little more than an empty shell.

Democratic lawmakers refuse to propose an alternative plan unless Bush abandons his demand for replacing part of Social Security with private accounts.

But one widely discussed alternative remains on the table: imposing Social Security taxes on earnings that exceed $90,000 a year, the current ceiling on income subject to payroll taxes.

That one change would affect 6 percent of all workers, the very highest earners, but actuarial experts estimate it would raise almost enough money to eliminate the projected shortfall without needing to cut benefits.

The two competing approaches — concentrating future benefits on the poor versus raising taxes on the wealthy — are almost mirror opposites.

Under "progressive indexation," an idea developed by Robert Pozen, an investment executive in Boston, the role of Social Security gradually would shrink. Adding private accounts to smaller guaranteed payments for retirees, supporters said, would soften the blow from scaling back the system to keep it fiscally sound.

By contrast, raising the ceiling on payroll taxes would maintain the system's solvency just as well, if not better, while allowing it to pay out substantial retirement benefits to the 30 million more people expected to be added to the rolls from the ranks of the nation's aging baby boomers.

How would the president's "progressive indexation" work?

As envisioned by Pozen, a Democrat who served on Bush's Social Security advisory commission in 2001, benefits for about 70 percent of workers would be indexed, at least in part, to increases in consumer prices rather than to increases in average wages.

Because wages have been rising about 1 percentage point a year faster than prices, the change would mean retirement benefits for middle- and upper-middle-class Americans would climb in nominal terms much more slowly than now promised.

While the buying power of those monthly benefits, adjusted for inflation, would remain steady, they would replace smaller and smaller shares of a person's working wages.

As a result, people without other sources of retirement income would experience a sharp plunge in their living standards as soon as they stopped working. Compared with future working Americans, such retirees living during the same period would slip further and further behind.

In an interview last week, Pozen said today's retirement system is tilted unfairly in favor of the affluent. Not only do high-income people have longer life expectancies, meaning they collect Social Security benefits for more years, but they also benefit disproportionately from tax breaks for pensions and individual retirement plans such as the 401(k).

"Low-wage workers depend almost entirely on Social Security for retirement income," Pozen said. "Today, we have a very regressive system from the point of view of government support for retirement."

But Democratic opponents of Bush's approach said the issues of fairness and inequality could be addressed just as easily by raising taxes on the wealthy.

Josh Bivens, an economist at the liberal-leaning Economic Policy Institute in Washington, said Social Security revenues have been undermined because income inequality has increased dramatically in the past 20 years.

One consequence, he said, is that more wages are spilling over the ceiling on income subject to Social Security taxes. In 1983, when the ceiling was $37,500, 90 percent of wages and salaries were subject to taxes. By last year, 85 percent of wages were covered.

If the ceiling were raised to cover 90 percent of earnings, Bivens estimated, the government would cover about 40 percent of its projected shortfall over the next 75 years. Bush has not ruled out an increase in the payroll-tax ceiling, but most Republicans are adamantly against it.

"I know some rich people, and if you ask them whether they would rather have a tax increase or their benefits cut, they'll immediately say, 'Cut the benefits,' " said Rep. Bill Thomas, R-Calif.

Under Pozen's approach, however, benefits would be curtailed gradually for anybody earning more than $25,000 a year.

Peter Diamond, professor of economics at the Massachusetts Institute of Technology and a co-author of a rival plan to shore up Social Security, noted that about two-thirds of people older than 65 rely on Social Security for more than half of their income.

"The implications [of Pozen's approach] are wider and deeper and more important than appears on the surface," Diamond said.