Federal union leaders who want more political clout for the nation's 2.6 million civil servants believe they have more than enough votes on the senate Governmental Affairs Committee to win quick approval of a modified Hatch Act III.

The legislation, already cleared by the House would repeal most provisions of the 38 year old law that has kept federal employees from taking active roles in partisan political campaigns.

Although the Governmental Affairs unit has been slow to act on the Hatch Act changes, union nose-counters now believe they have 12 to 13 votes, on the 15 member committee. Two committee members, Sen. John L. McClellan (D-Ark.) and Lee Metcalf (D-Mont.) died and have not yet been replaced.

AFL-CIO unions have pushed hardest for the "reform" of the Hatch Act, which President Carter has promised to sign into law. Committee Chairman Abraham Ribicoff (D-Conn.) says he opposes the bill, but has not tried to block it in his committee.

Lobbyists favoring the Hatch Act changes - which would permit federal civil servants to run as candidates, raise money or manage partisan political campaigns - also doubt that opponents can or will muster the effort to mount a filibuster to block a Hatch Act vote.

The American Postal Workers union and "National Assocaition of Letter Carriers, with nearly 400,000 members, plan a legislative rally here next week to push for a quick, and favorable, Hatch Act vote. Postal workers usually get a good reception on Capitol Hill because of their numbers (600,00), the fact that they are located in every community in the nation, and because they are heavily unionized. An estimated 8 of every 10 employees is a union member.

Although many Republicans oppose the idea of changing the Hatch Act (fearing that the Democrats would turn the bureaucracy into a powerful new political arm), lobbyists believe they can count on the votes of committee members Jacob K. Javits (R-N.Y.), Ted Stevens (R-Alaska), H. John Heins III ;R-Pa.) and possibly even Charles McC. Mathias Jr. (R-Md.). Only three GOP committe members are considered questiion marks.

"We aren't talking this up," a top lobbyist said yesterday, "because we don't want to become complacent, or give our people the impression that this is in the bag. But we have gotten our act together and, if you ask, I would have to say we are cautiously optimistic" that the Hatch Act changes will go to the full Senate "no later than early Marh."

He said that "despite what you read in the press, the White House is still behind us, and we are counting on Carter's approval." If his prediction is correct, the nation will soon have a new group of sophisticated, well paid and strategically placed voters ready - or at least eligible - to get involved in partisan politics. That could turn the balance of power in some communities and states.

And The Month Isn't Over Yet! Consider this congratulations-I-think item from the newsletter put out by a Navy base in Florida: "The morale of the employees in the commissary during the months of December and January hasincreased to 10 per cent. Let's keep up the good work!!!"