The head of the biggest union of federal employes predicted yesterday that 300,000 workers, about a fifth of the non-postal civil service work force, may be fired this fall unless taxes are raised or Congress repeals or modifies the Gramm-Rudman-Hollings deficit reduction legislation.

Kenneth T. Blaylock, president of the 200,000- member American Federation of Government Employees, told reporters that he expects heavy layoffs to begin with the new fiscal year in October.

By the end of September, the Defense Department must cut its budget by 4.9 percent, and other agencies must cut theirs by 4.3 percent. But in the fiscal year that begins Oct. 1, agencies must cut budgets by 20 percent, making federal layoffs inevitable, Blaylock said.

He said deeper budget cuts required by the second phase of the law could wipe out 400,000 jobs, and that attrition could handle only about 100,000 of the cuts. AFGE represents more than 600,000 workers.

U.S. officials continue to maintain, however, that reductions in force are unlikely this year. They said they expect agencies to cut spending by controlling hiring, travel and overtime, offering early retirement and, in some cases, furloughing workers one day every two weeks.

Constance Horner, director of the Office of Personnel Management, said she could make no predictions about job cuts because she doesn't know how large the deficit will be. Horner, a former Office of Management and Budget official, called Blaylock's estimate of 400,000 federal job cuts next year "an exaggeration that will perhaps result in a lot of unwarranted fear" among federal workers.

That level of job cutting would hit the Washington area's 347,000 civil servants considerably harder than the series of layoffs during the first three years of the Reagan administration that put 2,900 federal workers out on the street.

At a breakfast with reporters yesterday, Blaylock said legislators who voted for the automatic spending cuts were "political cowards" who then went home and "blamed it on a president who isn't running for reelection."

The pro-Democratic AFGE has frozen contributions from its $250,000 political action fund. Blaylock, a former Defense Department worker who sits on the governing board of the AFL-CIO, said politicians who seek financial help from the union will be "interviewed" to see where they stand on budget-cutting programs.

He said he has asked union members to increase their voluntary contributions to the union's political war chest. The money will be used to help friends and "punish our enemies," he said.

Blaylock said that in the past the union has supported few Republican politicians but has endorsed Democrats ranging from Guam's nonvoting delegate to D.C. Mayor Marion Barry and presidential candidate Walter Mondale.

AFGE warns that pending cuts could hurt Social Security delivery programs and cripple meat inspection and efforts to control the nation's borders and air traffic.