Union membership among Prince George's county employes is expected to drop this week by more than a quarter, to 740 members, because the county's clerical workers voted to drop out of the American Federation of State, County and Municipal Employees.

A 168-to-60 vote last week by nonsupervisory employes of Local 2079 is expected to be certified by the Public Employees Relations Board this week. The union has represented those employes for six years.

After decertification, the employes will receive the 7 percent cost-of-living increase granted nonunion employes last July, according to County Attorney Robert W. Ostrom.

Discontent with AFSCME has been brewing since a 17-day strike in August 1980, said Wilson Murry, a county inventory manager who has been active in the effort to drop the union. He said the union has given them fewer benefits than nonunion workers obtained on their own. Except for two months last year, AFSCME members in the county have been without a contract since July 1980.

Union officials charged that the employes had succumbed to County Executive Lawrence J. Hogan's "union-breaking" tactics.

AFSCME representative Paul H. Manner said talks with the county broke down earlier this year when union officials were told that a 5 percent cost-of-living increase was the county's "last and final" offer. With nonunion employes receiving 7 percent, he said, this was "obvious union breaking" by Hogan.

But Ostrom said negotiations were still in an early, "routine" stage when talks stalled.

Two years ago county corrections officers left AFSCME, after several were fired following the 1980 strike. Last year, a petition to leave the union was defeated by school crossing guards.