Even before Schumer offered his analysis in an interview, the White House was downplaying Bush's statements about the earnings cap. Trent Duffy, a spokesman for Bush, said the president believes it is important to keep all options for dealing with Social Security's solvency on the table, but "that doesn't mean he has embraced any one of them."

By taxing the earnings of those making more than $90,000 a year — a figure that is adjusted each year in line with inflation — the Social Security system could go a long ways toward restoring solvency over the next 75 years. That would ease the need for any benefit cuts to bring the pay-as-you-go system into balance between incoming revenues and outgoing payments.

Greenspan's comments, in testimony to the Senate Banking Committee, provided some support to Bush on the private accounts component of his approach to Social Security but also gave ammunition to Democrats who have asserted that establishing investment accounts is, among other problems, unaffordable.

"I think the existing structure is not working," Greenspan told members of the Senate Banking Committee, declaring that private accounts would be "a good thing to do" but urging lawmakers to "start out slowly" and be wary about the trillions of dollars in additional federal borrowing that might be necessary.

"If you're going to move to private accounts, which I approve of, I think you have to do it in a cautious, gradual way," Greenspan said.

The comments were reminiscent of those Greenspan made just over four years ago, when he endorsed Bush's goal of cutting taxes on the theory that the government should gradually reduce its budget surpluses.

On Wednesday, Democrats had been hoping that Greenspan, who has long advocated steps to bring down the federal budget deficit, would wave a red flag about the potentially huge borrowing that Bush's Social Security plan is likely to entail.

He avoided doing so. Greenspan said his warning about the risks of "large" additional borrowing applied to amounts of "more than $1 trillion" over ten years.

White House officials have estimated that Bush's plan would require borrowing about $754 billion over the next decade. That figure, however, reflects Bush's strategy of delaying introduction of the accounts until 2009 and to phase them in over a number of years, a step he has said was intended in part to hold down the costs.

Vice President Dick Cheney has acknowledged that the costs would be in the trillions of dollars in subsequent decades once the program is fully up and running, and some outside experts estimate that total borrowing could exceed $4 trillion over the next several decades.