The Senate Governmental Affairs Committee tomorrow is expected to propose what would be the biggest early retirement offer ever dangled before American workers: A 90-day period in which as many as 700,000 of the government's 2.8 million civilians could leave early and start getting immediate pensions.

If approved, the plan by Chairman William V. Roth (R-Del.) and Sen. Ted Stevens (R-Alaska) will be sent to the Senate Budget Committee as a money-saving inclusion to the budget for the fiscal year that starts in October.

The plan would waive normal civil service age and service requirements for a three-month period. During that time 500,000 workers who are not now eligible to retire could leave with a pension. Their benefits would be reduced 2 percent for each year they were under age 55.

Normal federal age-service requirements to qualify for immediate pension are age 55 with 30 years' service, age 60 with 20 years' service, and age 62 with five years' service. Benefits are based on length of service and salary.

During the early-out period, age-service requirements would be waived for anyone who is within five years of being eligible to retire under regular rules. That would allow them to retire much earlier than they had planned.

Also, the plan would create four new categories of eligibility for early retirement: anyone with 25 years' service; anyone age 50 with 20 years; employes age 55 with 15 years or those who are 57 with five years.

Roth says the early out -- coupled with a limited three-year hiring freeze -- would save about $2.5 billion. His proposal would allow the president to exclude up to one quarter of the eligible employes from taking advantage of the early out (to prevent a massive exodus of hard-to-replace workers), and it would give the president authority to allow employes to replace some workers who took early outs.

Unions are suspicious of the plan. They think it is designed to slash the civil service work force and allow more contractors to take over government work. They also believe that dangling the early out, which is popular with workers, could make it easier for Congress to go along with major cuts in federal programs.

Roth hopes to sell the plan on the basis of its savings features. But he also says it will allow federal agencies to make budget cuts in coming years without having to fire younger employes who lack seniority.

The early-out plan is a long way from becoming reality. But this new strategy -- making it part of the budget process -- has pumped new life into a scheme that seemed dead as recently as two weeks ago.