Leaders of 50 federal postal unions, retiree groups and professional organizations have tentatively agreed to coordinate political contributions to make sure they don't give money to politicians who back federal pay or pension freezes, or Social Security coverage for civil servants.

The organizations, ranging from big independent and AFL-CIO unions to small professional groups, rarely get togather. But they did last week on Capitol Hill, where backslapping replaced back-stabbing.

Although they represent a wide range of government employes--from postal clerks to scientists--they are united in fear of President Reagan's plans to cut costs by delaying raises and contracting out more federal services, and of the bipartisan push to put new government workers under Social Security.

Despite the fact that many of the groups have been losing members in recent years, most of them have policital action committee (PAC) warchests--supported by special member contributions--that are at record-high levels. In the last election they donated millions of dollars to candidates (mostly Democrats) for Congress.

While there has been some PAC coordination between some AFL-CIO affiliates, most of the groups dole out political contributions independently. But they hope to set up some kind of system to be sure that no money goes to any politician who doesn't back them on basic pay and pension issues.

"We've been too soft-hearted, especially where some Democrats are concerned," one union lobbyist said. "We have sometimes supported a guy because he was a good guy, or backed us on key issues. We sort of looked the other way when some of them strayed from our position. If we can keep this thing togather and keep the principals (union presidents) from posturing or fighting or making cracks at each other . . . we can become quite a force."

Meantime, federal unions headquartered in the Washington area have been getting a large number of telephone inquiries in the last two weeks from workers requesting membership applications. Although metro Washington--with 342,000 federal employes--has the nation's largest concentration of government workers, union membership here has traditionally been low. It is estimated that fewer than 40,000 employes here belong to government unions. Resignations: Future meetings of the federal pay advisory panel will be a trifle management-oriented. All of the union representatives on the panel (which is supposed to meet with government officials to discuss pay matters) resigned from it last week.

The union leaders are protesting the administration plan to skip this October's pay raise (they say feds are due a 22 percent catchup with industry), and proposals to revise the system government uses to keep pay on par with industry.

Leaders of AFL-CIO's Public Employes Department; American Federation of Government Employes; National Federation of Federal Employes and National Treasury Employes Union quit the panel. They say they won't come back until the government is prepared to talk "realistically" about the pay situation.

Reductions In Force: Office of Personnel Management Director Donald J. Devine has agreed to consider complaints of area Republican politicians about OPM proposals to change federal RIF rules. OPM wants to give more job protection to employes based on performance and to downgrade the value of seniority when agencies decide who gets RIFfed.

OPM would rearrange protective rules so that employes with better job ratings would be sparred RIFs, even if it meant firing more senior workers with lower job performance ratings.

Devine met last week with Reps. Frank Wolf (R-Va.) and Marjorie Holt (R-Md). Both oppose RIFs, but say that if they come workers with seniority should be given the greatest protection from firing.

Also attending the meeting were representives of the Federal Managers Association, Senior Executive Association and the Organization of Professional Employes-Department of Agriculture .

OPM plans to publish proposed RIF rules changes shortly in the Federal Register. Agencies (and individuals) would have 60 days to comment on the proposals. Based on suggestions and comments OPM could modify its proposal, or implement them after responding to complaints.