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

The nonpartisan Congressional Budget Office 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 about two-thirds the $3.7 trillion estimated in March by the bipartisan trustees who oversee Social Security, the aides said yesterday. 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, as to 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.