The Senate Democratic Caucus, in a resolution opposing many of the major Social Security cuts proposed by President Reagan, said yesterday that Congress could "protect the soundness of the Social Security system" adequately for the present simply by authorizing the large trust funds to borrow from one another.

The resolution, which also urged President Reagan to drop the idea of delaying Social Security cost-of-living increases, seemed to dampen chances that Democrats and Republicans will join in a bipartisan bill to shore up the system. But Finance Committee Chairman Robert J. Dole (R-Kan.) said he still hopes something can be worked out.

Meanwhile, the Committee for Economic Development (CED), an organization of business and education leaders, issued a report urging major changes in Social Security and the nation's private pension plans to guarantee their soundness over the long run.

For Social Security the CED called for gradually raising the age of retirement with full benefits from 65 to 68 but allowing early retirement with reduced benefits at 65.

The CED also proposed curbing increases in the annual cost-of-living adjustment by basing it on a special inflation index for the elderly, or on the increase in wages in the economy as a whole, whichever is less; making Social Security contributions paid by workers tax deductible but then taxing the benefits when received at retirement; bringing all new federal and non-federal government workers into Social Security and abolishing the earnings test for Social Security recipients.

For private pensions the CED proposed making employe contributions tax deductible, allowing workers who take new jobs to switch their private pension credit to individual retirements accounts (IRA) or to private insurance under certain conditions and raising the allowable IRA contribution levels.

Earlier yesterday, William Niskanen, a member of the Council of Economic Advisers, and Congressional Budget Office Director Alice M. Rivlin told a congressional committee that, under the most likely economic developments of the 1980s, the Social Security trust funds will be solvent until at least 1988 if interfund borrowing is authorized.

But both emphasized that, if the economy turns out only a little bit worse than expected, things will begin to go sour around 1984 or 1985 and therefore, as Rivlin said, it would be "prudent" to build up the trust funds either by increasing revenues or cutting benefits.