An election-year duel over President Bush's proposal to revamp Social Security (search) could flare up again Monday with a congressional report that says the giant pension program's long-term prospects may be better than previously thought.

The nonpartisan Congressional Budget Office (search) will say that over the long run, Social Security remains unsustainable at the current levels of benefits and payroll taxes that finance the system, said Capitol Hill aides who spoke on condition of anonymity.

Yet the report is expected to project that the system's shortfall over the next 75 years will be only about two-thirds the $3.7 trillion estimated last March by the bipartisan trustees who oversee Social Security, the aides said Friday. The discrepancies are due to differing economic and other assumptions, they said.

This means Social Security's 2042 insolvency, as projected by the trustees, would occur about a decade later, the aides said. The aides differed, however, about whether the budget office report would predict a later insolvency date or whether the data would simply support that conclusion.

A projection of a more robust Social Security could provide ammunition to opponents of Bush's idea of letting people buttress their benefits by diverting some of their payroll taxes into private investment accounts. Bush has not said how those accounts would be financed, and the plan has drawn opposition from many Democrats as a threat to the pension program, which had 47 million beneficiaries last year.

"If CBO is correct, we don't need severe, radical changes to Social Security," said Robert Greenstein, executive director of the liberal Center on Budget and Policy Priorities (search).

Part of the reason for the budget office's more optimistic outlook, however, is its assumption that people will be due a slightly lower level of benefits over the long range than the trustees assumed, the officials said.

This is due to budget office assumptions about economic and social trends, they said, and could help supporters of Bush's plan argue their case.

"If all the left can do is cheer the fact that people are getting lower benefits, it shows me ... how weak the case is to prop up the status quo," said Daniel Mitchell, an economist at the conservative Heritage Foundation (search).

The report is expected to examine Social Security differently than the program's trustees do in their annual reports. For example, it will show the benefits likely to be earned by different categories of workers, such as by men and women, the aides said.

The trustees projected in March that Social Security's problem could be rectified by increasing the tax that supports the system by 1.89 percent. But the congressional report will say an increase of only about 1 percent is needed, two aides said.

Melissa Merson, spokeswoman for the Congressional Budget Office, would not discuss the report.