Social Security Administration employes from around the country whose jobs may be endangered plan to converge on Washington tomorrow to protest a proposal to eliminate 17,000 positions during the next five years and shut some Social Security offices. The American Federation of Government Employees union estimates that several hundred workers will participate.

Earlier this year the president proposed gradually cutting employment at SSA and other agencies as part of the plan to trim federal costs and streamline government operations.

In addition to the proposed job cuts, which would have to be approved by Congress, the agency is undertaking a review to see if some of its functions could be handled better, and more cheaply, by private contractors. One element slated for that cost-comparison review is the 5,000-employe national computer center in Baltimore.

Although it is technically a part of the Department of Health and Human Services, the Social Security Administration is a kingdom of its own, with 70,000 workers who supply benefit checks to one of every six Americans.

It has more employes than either the Justice Department or the Transportation Department -- more than the departments of State, Education, Housing and Urban Development, and Energy combined. More than 20,000 workers are assigned to SSA's Woodlawn central office just outside Baltimore, making it one of the largest white-collar facilities in the nation.

For the last few months, SSA employes have been telling reporters and members of Congress about internal plans to cut jobs and contract out work. Officials say that the contractor studies are going on throughout the government but that no conclusions have been reached.

Reps. Michael D. Barnes (D-Md.) and Vic Fazio (D-Calif.) have obtained internal documents from SSA and the Office of Management and Budget and plan to make them public tomorrow when SSA employes, many of whom are traveling on chartered buses, arrive on Capitol Hill.

Members of Congress are almost as jittery as SSA employes about the rumors of cutbacks. Any mention of changes in Social Security benefits, procedures or services sets off political alarm bells because of the potential impact on so many people, most of them old, disabled or dependent.

Sen. Lawton Chiles (D-Fla.) has added language to the budgetary legislation for the departments of Labor and Health and Human Services that would bar SSA from closing any local or district office and would give the agency about 1,000 new employes.

The administration will have a hard time selling Congress and the public on any plan to cut employment, unless it can make its case that the action will improve service. People who oppose the plan will do their best to convince Congress, the media and the public that any staff or office reductions would harm service.