Federal and postal unions that have been slowly building political war chests through direct-mail pleas, raffles and sales of anti-Reagan items (buttons and T-shirts) at meetings and picnics may soon be able to tap an almost unlimited fund source: Federal payroll deduction.

Although nearly half the 2.8 million member U.S. work force is represented by unions (80 percent of the postal service is union) federal, postal and retiree groups find political fund-raising a real chore.

But things could change if agencies grant voluntary PAC (political action committee) checkoffs as part of contract agreements with unions.

The American Federation of Government Employees has just negotiated a tentative checkoff agreement with the D.C. Department of Employment Services. It allows the 600-plus employes in the department who are repesented by the union to make payroll deductions to AFGE's political fund. The minimum deduction is $26 a year. (Although the majority of AFGE members are federal employes, it does represent some D.C. government workers).

Union PAC funds are kept separate from other accounts and cannot, by law, be supplemented by members dues or fees. PAC funds are used to make donations to campaigns, buy tickets to fund-raisers and otherwise help friends and hurt political foes.

Most U.S. unions and retiree groups have political funds. But what income they get comes in too slowly to suit their leaders.

AFGE's new checkoff is believed to be a first for a union representing federal employes.

Employes at the Veterans Administration hospital in Syracuse, N.Y., have political action contributions deducted from their paychecks thanks to an agreement negotiated by the Service Employees International Union. The American Federation of State, County and Municipal Employees and American Federation of Teachers also have similar agreements, mostly with local governments.

The potential for PACs in the federal service is almost unlimited. The average federal salary nationwide is over $21,000. The typical white-collar fed here earns $26,000.

Government workers are angry over budget cuts, rifs, retirement cuts and the "antibureaucrat" attitude of Congress and the administration.

If unions can tap the federal payroll--via voluntary payroll deductions--for political contributions, they could become a major political force in time for the 1984 election.