Federal and postal workers hired since last Jan. 1 would be put into a new pension program that would base their retirement benefits on Social Security, a modified civil service retirement package and optional tax-sheltered investments under a Senate plan that will be outlined in a few days.

The 2.6 million U.S. workers already covered by the regular civil service retirement plan would not be required to join the new system developed by Sen. Ted Stevens (R-Alaska). But current feds would be given the option to buy into the new plan if it is cleared by Congress and the White House.

Stevens has been waiting for Senate Republicans to pick a new majority leader before outlining his new retirement package, which is separate from planned White House "reforms" of the current pension program. The Senate leadership battle was settled yesterday when GOP senators selected Kansas Sen. Robert Dole over Stevens by a 28-25 vote.

Aides to Stevens have declined to spell out the plan until it is formally presented to the Senate. But from other sources this column has learned the plan would:

* Automatically take in everybody hired by the government since Jan. 1.

* Encourage government workers under the regular civil service system to come voluntarily into the new plan by allowing them to transfer funds from their retirement accounts into the new system. In return, they would be allowed to set aside part of their salary into tax-deferred private sector investment plans.

* Establish in government something akin to the 401(k) tax deferral plans that are widely used by private firms. Those plans allow workers to put a portion of their salary directly into one of several investment options. Money invested is not taxed until workers start collecting it as part of their retirement benefits or withdraw it for emergency reasons.

* Allow workers to collect full Social Security benefits at age 65, and to take partial benefits as early as age 62.

* Employes could continue to retire from the civil service as early as age 55, but with a reduced benefit based on smaller contributions they would be making to the civil service retirement program.

Stevens' plan is separate from a coming administration proposal to overhaul the current civil service retirement program.

Although the White House has not approved a final version of its retirement reform package, it is expected to call for higher contributions from employes (who now put 7 percent of their salary into the retirement fund) and to reduce future cost-of-living raises for retirees. Those raises are now linked to the rate of inflation.

The White House plan may or may not call for raising the retirement age for some current civil servants. But the administration is eager to ensure that any new plan for new government employes require that they work until age 62 or 65 to get full benefits. Any adminstration plan would need congressional approval before being implemented.

Government employes can now retire at age 55 with 30 years service with an annuity equal to about 56 percent of their final salary. Full benefits, 80 percent of salary, are reached only after about 42 years of federal service.

Two years ago the administration proposed reducing benefits 5 percent a year for each year the employe was under age 65. That plan flopped in Congress, because of an onslaught from federal and postal unions.