Reagan administraton officials and federal union leaders will spell out their positions tomorrow on new early retirement proposals in a one-day hearing before the Senate Governmental Affairs Committee.

Legislation currently being studied would make it possible for an extra 200,000 U.S. workers to leave government service on some form of pension between July and December.

The bill, introduced by Sens. William V. Roth Jr. (R-Del.) and Ted Stevens (R-Alaska), would suspend civil service age and service rules from July 1 through the end of the year, making it possible for some to retire as early as age 43.

Reagan administration officials are expected to urge that the period be shortened, perhaps to three months, and that eligiblity be narrowed to employes with 25 years of service (regardless of age) or to workers with at least 20 years' service who are 50 years old.

Federal unions will ask that replacement hiring be allowed, so that vacancies created by early retirement could be filled. Congressional sources estimate that up to 65,000 employes might take advantage of early retirement if offered. Normally about one-third that number of employes would retire in July through December.

Unions are also anxious to protect vacant federal jobs from being turned over to the private sector. They also oppose early retirement penalties in the Roth-Stevens bill.

The proposal would double the number of workers who could retire this year.

Roth and Stevens say the idea is to encourage older employes to leave so that newer, younger workers -- those most likely to be laid off -- can keep their jobs as agencies are hit by budget cuts required by the deficit reduction act. The Roth- Stevens bill also would impose strict hiring restrictions on agencies for the next five years.

Under Roth-Stevens workers could retire at any age after 25 years' service, at age 50 after 20 years' service, at age 55 after 15 years' service or at age 57 after five years of service. Annuities would be reduced 2 percent for each year the retirees were under age 55. Job Mart

The Marine Corps is looking for an air-conditioning mechanic, Wage Grade 16. Call Ann Brant at 694-1046.

Health Care Financing Administration needs a Grade 12 public affairs specialist. Call Ella Fawley at (301) 597-2985.

Internal Revenue Service wants procurement analysts, GS 11 through 13. Call Jeff Page at 535-4790.

Defense Nuclear Agency is looking for a GS 11/12 personnel management specialist. Call Bill Embry at 325-7591.

Bureau of Engraving and Printing has an opening for two GM (merit pay) management analysis officers. Call Howard Katz at 447-0459.

Federal Home Loan Bank Board needs clerk-typists, GS 4, and secretaries, GS 5/6. Civil service status required. Call 377-6060.

Bureau of Alcohol, Tobacco and Firearms wants a temporary, GS 3/4 clerk-typist in Rockville. Call 377-9861. Meetings

The American Association for Affirmative Action will hold a professional development seminar May 23 at the Capital Hilton Hotel. Speakers include officials from the Labor Department, Equal Employment Opportunities Commission and the District government. Call Eugene Walton, 287-5497.

Congressional Budget Office Director Rudolph G. Penner will speak tomorrow evening at the American University's Kay Spiritual Life Center. He will talk about the congressional budget process. For information, call Susan McCollum at 885-1200.