Publicity blitzes being launched against proposals to trim outlays for government employe pay and pension programs are turning into a financial bonanza for the advertising industry.

What's happening is that unions representing the nation's 2.6 million federal and postal workers are spending big bucks trying to bolster the image of civil servants and lobby with ads against congressional and White House deficit-cutting plans.

The unions want to force budget trimmers to consider cuts in defense spending and corporate tax breaks rather than the federal payroll.

The unions want to convince the American public and Congress that government workers are doing necessary jobs, and doing them well. If the current proposals are passed, the unions predict, pay will be frozen or cut, benefits will be further reduced, jobs will be lost and the civil service will become nothing more than a political mouthpiece for whichever party is in power.

An antibureaucrat campaign was started by the Carter administration and has been continued with a vengeance by the Reagan administration, the unions say. The result, they contend, is that the public has been wrongly convinced that civil servants are an overpaid, underworked collection of drones in green eyeshades who shuffle papers, grumble, oppose change and retire on enviable pensions.

Hampered by no-strike laws, low dues structures and declining memberships, the unions are trying the Madison Avenue approach as never before. For example:

* The National Association of Letter Carriers is preparing to spend $1 million and has hired the Hannaford Co., the old public relations firm of White House aide Michael Deaver, to help with a media blitz aimed at scuttling White House plans to trim U.S. pay and retirement benefits.

* The American Federation of Government Employees has already begun a $300,000 advertising campaign designed to remind taxpayers that the first man on the moon, an employe of the National Aeronautics and Space Administration, was a civilian government worker; that federal workers patrol the nation's borders and guard its water supply, and that all these employes pay taxes, too.

* The American Postal Workers Union has been buying advertising time to explain what its more than 200,000 members do. The union is concerned that plans to raise employe contributions to the retirement fund could wipe out pay raises it has negotiated for its members.

* The Public Employees Roundtable, a coalition of 22 federal worker organizations, is sending suggested pro-civil servant editorials to newspapers and television stations, and is encouraging government workers and other citizens to help finance its campaign.

* The National Federation of Federal Employees, National Treasury Employees Union and others are also directing more time and dollars to the media. The National Association of Retired Federal Employees hopes to repeat its success of two years ago, when it raised $1 million in less than one month from its members, whose incomes average $12,000 a year, to help stave off cutbacks in retirement programs.