Many members of Congress -- and not just those from big civil service communities -- are predicting defeat for President Reagan's proposal to trim federal wages by 5 percent next January.

The potential impact of the cut -- it would cost the Washington area alone $2 million a day in wages -- and plans to trim federal retirement benefits, have prompted many of those members to sign up with the Federal Government Service Task Force, the congressional equivalent of a civil service caucus.

The task force now has 42 members -- 40 from the House and two from the Senate, all Democrats -- but leaders expect some Republicans to join later. All task force members come from areas where federal workers and retirees make up a substantial voting block.

The cochairmen, Mike Barnes (D-Md.) and Vic Fazio (D-Calif.) say the group may be a major force this year in blocking or modifying Reagan budget cuts targeted at U.S. workers.

Thanks to the 3 1/2 percent federal pay raise that went into effect last month, along with a 2.7 percent increase for postal workers, the U.S. payroll in this area will exceed $1 billion next month. That amount does not include annuity payments to more than 100,000 federal and military retirees here who also got a 3 1/2 percent cost-of-living adjustment in January.

The federal salary cut would hit nearly 2 million U.S. civil servants in nearly every town in America. The biggest impact would be in the Washington area, which has nearly 350,000 U.S. employes.

But the cuts also would be a major factor in big cities and many small towns where the ratio of federal to private sector workers is even higher than in Washington. The federal payroll now exceeds $1 million a month in nearly 300 communities nationwide.

Prices here go up for everybody when feds get a raise. But nobody is betting that they would go down if feds take a pay cut. Last time a pay cut was tried was in the 1930s, when the nation was in the midst of the Great Depression.