Imagine watching a news conference of a candidate for mayor, Congress or president and later seeing the candidate in uniform putting mail in a box, processing a passport or Social Security claim, or auditing income tax!

Although none of the above is likely to happen, it could, under a bill the House Post Office and Civil Service Committee is expected to pass Tuesday.

The plan, by Rep. William L. Clay (D-Mo.), would give full political rights for the first time in memory to nearly 3 million federal workers, from Veterans Administration clerks to air traffic controllers, meat inspectors and Defense Department contract managers.

Clay's bill, strongly supported by federal and postal worker unions, would revamp the Hatch Act, which was passed in 1939 by a Congress that claimed that the Roosevelt administration was guilty of arm-twisting federal workers. In addition to barring politicians and supervisors from putting the arm on employes -- for donations of time or money for partisan purposes -- the law also bans employes from taking active roles in partisan political campaigns, as advisers, fund-raisers, vocal supporters or candidates.

Federal and postal unions can't strike and, except in the Postal Service, can't bargain over pay and fringe benefits. They have pushed for Hatch Act revision almost since the ink was dry on the law.

Their high-water mark came more than a decade ago. The Democrat-controlled House and Senate passed a revision bill only to have it vetoed by Republican President Ford.

Their low-water mark came a short time later when Democratic President Carter, who favored Hatch Act reform as a candidate, let it be known that he would not be unhappy if the Senate, then controlled by Democrats, sat on a Hatch Act reform bill. It did.

This year, however, Hatch Act revampers think they have the momentum and votes. Clay's bill had more than 100 cosponsors when it was introduced last week. Committee approval is all but assured Tuesday. The House is expected to okay it, and key Senators have said they will begin work shortly. Although Senate passage this session is doubtful, the bill could be approved in 1988, an election year, and become law even over a presidential veto.

Under Clay's bill, off-duty federal workers could run for office, manage campaigns or raise money for a candidate or political party. As federal employes but minus job titles, they could endorse candidates.

The potential impact of Hatch Act changes, especially in this area with its 350,000 federal employes, is staggering.

Federal workers are among the nation's best-paid, best-trained, best-educated workers. The typical federal employe earns $27,000 a year outside Washington (here the average is more than $32,000), has more years of education than most people, and had to pass qualifying and background tests to get his job.

Whether they are cogs in the wheel or major policy implementers, federal employes have jobs that often carry more power and responsibility than do those of their neighbors.

Arguments for and against revision are obvious: Isn't it a sin to make second-class citizens of these workers simply because they get federal paychecks? Or, what if this leads to a politicization of the civil service that makes today's office-politics buddy system seem like amateur night?

Love it or hate it, Hatch Act reform may be around the corner.