Two lawmakers from Maryland yesterday blasted a Reagan administration proposal to slash the number of employes at the Social Security Administration, saying the move would mean a loss of about 5,000 jobs in the Baltimore area, where the agency headquarters is located.

"The president made a commitment not to cut benefits" so instead he is making "backdoor" cuts in services, Sen. Paul S. Sarbanes (D-Md.) said at a news conference organized by the Social Security Management Association, an organization of supervisors of the agency's 1,300 offices.

Rep. Barbara A. Mikulski (D-Md.) said a reduction in the Social Security staff would increase the time it would take to serve claimants.

The Office of Management and Budget has proposed reducing the agency's 75,000 workers by 17,000 over five years, as a modern computer system becomes fully installed. Mikulski said that the cuts would eliminate 5,000 jobs in the Baltimore area.

Sarbanes said that the new computer has been "inadequate," but that even if it were running efficiently, the modernization should affect far fewer employes.

Sen. David Pryor (D-Ark.) said agency staffers are the "only human link" Social Security recipients have with the government. "This country has not reached the point where the claimants should go into an office and try to explain a problem to a computer," said Pryor.

Pryor and others have introduced bills to make the Social Security Administration an independent agency, governed by a board, with a commissioner appointed for a five- year term. Pryor said the proposal got little support during the last session of Congress, but added that he expects more support because of both proposals to reduce the agency staff and controversy over who will head the agency.