President Carter's chief personnel adviser says Congress must supply answers to some important questions before the White House will endorse mandatory social security coverage for 2.6 million civil servants.

On Monday, the House Ways and Means Committee voted to extend mandatory coverage to federal, state and local government workers beginning Jan. 1, 1982. Although the committee promised that the plan would not cut benefits for federal and postal workers or mean a big boost in the price they pay for retirement benefits, there is nothing in writing to spell out how - or ever, if - that promise is to be kept.

Rep. Joseph Fisher (D-Va.) who opposed the mandatory social security coverage, lost that fight, but he did persuade the committee to make the nobody-will-get-hurt promise.

Details for working out the incredibly complicated integration of social security with the civil service staff retirement system are supposed to be ready in 1980. That is two years before universal coverage becomes law - if both the House and Senate go along with the plan.

In a statement yesterday, Civil Service Chairman Alan K. Campbell said he is not "fundamentally opposed" to the mandatory coverage, which is already standard practice for most private workers, and for about half the 12 million state and local government employees.

"My concern," Campbell said, is that procedures be worked out that will guarantee that government workers won't lose benefits already pledged to them, and that they not be hit by "unreasonable" costs under the dual social security-civil service coverage.

Federal workers now contribute 7 per cent of their total annual income into the civil service retirement fund, which pays much more generous benefits, and permits earlier retirement, than social security. The government agency of each employ matches that 7 per cent contribution rate.

Workers under social security now pay 5.85 per cent of their first $16,500 of income, and that amount is matched by their employers. The contribution rate will go up to 6.05 per cent of the first $17,700 next year' but that is still less than federal workers now pay.

Campbell says that if the dual coverage were introduced next year, for example, "the typical federal employee would be making the current contribution of $1,239 into the civil service retirement fund, plus an additional $1,071 into social security and agency." [TEXT OMITTED FROM SOURCE]

(Ways and Means Committee sources told this column Monday night that federal workers would not have to make double contributions. What they said is that employees would pay the same rate as nonfederal workers into the social security fund, plus a differential of about 1.15 per cent to maintain current civil service benefits.

Upon retirement, federal employees would get two checks each months, one from social security and one from the civil service retirement fund. That concept has not been written out in legislative form yet, and Campbell obviously wants to see it in black and white before he recommends that the President support mandatory coverage.)

Campell's statement said "I do not believe that anyone seriously intends to require this kind of dual coverage and dual cost - not that anyone seriously advocates using the assets of one system of shore up the other. But rather, I believe, it would be possible after adequate study of both the benefits and the financing of the two systems, to develop a means to achieve a merger."

So Congress still has a lot of answers it must supply to government workers, their unions, and the White House, before universal coverage gets their support. It would soothe a lot of nerves in the bureaucracy if it comes up with those further in pushing the dual coverage.

Retirees: Ways and Means Committee sources say that none of the million-plus retired federal workers or their survivors has any worries about the universal coverage. Those people will continue level (with regular cost-of-living adjustments) and that money will come from the civil service retirement fund, not from social security, they say.