Federal union leaders and House Democrats hope to add 2.6 million of the nation's best-paid, best-trained and most strategically located workers to the ranks of the politically active within the next six months.

The new recruits would be civil servants and postal employees who - if the congressional timetable holds - will be free of all partisan political restrictions by midsummer and ready to campaign, contribute to campaigns and run as candidates by fall.

Since 1939, career federal workers have been severely limited as to their political activities because of a collection of administrative rulings and decisions that took legal form in the so-called Hatch Act. Almost since that time Democrats in Congress and the AFL-CIO have been trying to reform or repeal the Hatch Act, and unless somebody flubs it very badly this should be the year the Hatch Act is buried following major surgery.

The issue goes beyond mere inhouse bureaucratic fringe benefit improvements because of the size - and importance - of the federal work force and its important impact in nearly half the nation's congressional districts and states.

Although administration officials are still trying to figure out President Carter's exact stand on reform of the Hatch Act, he is on record as favoring major changes in the law.

Both the Senate and House (controlled by Democrats) passed bills last year - by big margins - that would have removed most of the partisan political restrictions on federal and postal employees. President Ford - as Congress knew he would - vetoed the bill on grounds that it was too far reaching and would, he said, subject federal employees to political arm-twisting and possibly make the American people hostages to their public servants.

Congress didn't override the veto, partly because of overwhelmingly national newspaper editorial opposition to repealing the Hatch Act, and also because everyone anticipated an easier time this year with Carter in the White House.

Prime mover behind Hatch Act reform is Rep. William Clay (D-Mo.). His House subcommittee plans two days, of hearings and then quick action next week on changes in the law. Clay's bill is similar to one he introduced last year that the House cleared.

Virtually all AFL-CIO federal and postal unions back Hatch Act changes for two reasons. The one most often stated is that it is unfair - and wasteful - for 2.6 million of the nation's most educated workers to be denied political citizenship rights simply because they get a government pay check.

Perhaps more important from the union viewpoint is the tremendous clout repeal or reform of the Hatch Act would give them on Capitol Hill, in statehouses and with the White House. Most of the unions endorsed Jimmy Carter last summer and eight of 10 candidates they have backed worked for and helped financially have been Democrast.

Federal workers now may vote (of course) and help partisan political candidates in a low-key, behind-the-scenes role. They cannot - under the Hatch Act - take active roles in campaigns, or run as either Democratic or Republican candidates.

Permitting government workers to take the same roles - including contributions and off-the-job fun-raising - as other citizens could tip the balance of political power in many key states. The federal employee population in some key states:

Alabama, 57,096; California, 299,304; Colorado, 47,252; Florida, 77,898; Georgia, 74,150; Hawaii, 25,319; Illinois, 108,001; Indiana, 41,335; Maryland, 65,097; Massachusetts, 57,673; Missouri, 66,214; New Jersey, 70,834; New York, 174,255; North Carolina 41,961; Ohio, 94,454; Oklahoma, 50,059; Pennsylvania, 133,074; Tennessee, 52,931; Utah 36,014; Virginia, 77,830; Washington State, 56,961 and Wisconsin 26,997. Maryland-Virginia figures do not include the metropolitan Washigton area, which has 350,000 federal workers.

You don't need a pocket calculator to figure that in most of those states - and many not mentioned - a heavy turnout of helpers and contributors from among the federal ranks could be decisive in many local and national elections.

Patrick J. Nilan of the American Postal Workers Union, one of the towns' sharpest federal lobbyists, figures the Hatch Act reform bill could be out of the House by May and possibly clear of the Senate by June. President Carter has indicated he would sign legislation and that could put the newly freed voters into the political mainstream by early fall, and certainly in time to have an impact on the 1978 congressional elections.

Department of Single Parents: Health, Education and Welfare may set up a special division to study the problems facing one-parent families. It could come up with proposed legislation for tax-breaks for day-care, recommendations on the pros and cons of flexible hours and the 4-day week for single parents and the like.

In the January-February issue of "The Single Parent" Capitol Hill correspondent Bob Westgate says that a pre-election adviser to Jimmy Carter suggested that HEW tackle the single parent problem and establish a separete unit for it.That adviser, says Westgate, was Washington lawyer Joseph A. Califano Jr., who now heads HEW.