Capitol Hill insiders believe there is a good chance Congress will not complete final action on the controversial Social Security bill until early next year.

The question of when that bill becomes law is vital to thousands of federal retirees and public employees. It will determine the effective date for all key items in the bill, including the important and complex provision that would cut back Social Security auxillary benefits for many public retirees, and virtually all the 8 million federal, state and local government workers now on the payroll.

The Social Security bill approved by the Senate would make major changes in the law that now permits U.S. and public retirees to draw a full pension and also to qualify for full dependent or survivor benefits based on the Social Security entitlement of a spouse.

Under present law, federal workers who are 62 years old may draw, when they retire, one half of their spouse's Social Security benefit as a dependent if the spouse also is 62. If the spouse predeceases the federal retiree, the survivor (at age 60) may receive all the spouse's Social Security benefit.

(The exception is when the federal retiree already is getting a Social Security pension, in which case the retiree receives only the larger of the two Social Security benefits). That would not change for persons already drawing those benefits, or who have qualified and applied for them.

But, after the effective date of the bill (if the Senate version becomes law), future retirees would have their dependent for survivor benefit (based on a spouse's entitlement) reduced dollar-for-dollar in terms of their federal or public pension.

If the Senate version were to become law this month, the effective date for offset would be Dec. 1. Persons applying for survivor benefits or dependent benefits after that date would be subject to offset. Thousands of federal and public retirees who meet the age requirement and whose spouses are entitled to Social Security benefits already have applied for them, hoping to beat the deadline if the Senate bill becomes law.

Federal retirees who already qualify for dependent benefits from a spouse's Social Security get one-half of it, up to $182 a month. Retirees who get widow's benefits from Social Security now get - and will keep getting - up to $401 a month. Those are the amounts that would be subject to offset for new retirees, or persons applying for benefits if the Senate bill becomes law.

The House-passed Social Security bill does not contain any offset language. Federal and postal unions and retiree groups are fighting to kill the Senate bill, or at least delay its effective date. Meantime, they are hoping the conferees cannot agree before the end of this month, so that retirees who otherwise are eligible would have until Jan. 1 to file for and get full dependent benefits.