The man who today begins steering the President's civil service "reforms" through the House has a deep understanding of the job jitters that have hit many bureaucrats since Carter announced his proposals to streamline the federal hiring, tenure and firing sysytem.

Post Office-Civil Service Committee chairman Robert N.C.; Nix (D-Pa.) has his own job security problems at the moment. He faces a very tough Democratic primary fight in Philadelphia in May. Winner of that primary in the heavily black, Democratic district is a 20-to-1 favorite to win the November election against whoever the Republic contender is.

Nix, whose committee will handle the civil service shakeup plan, won the primary in 1976 by only 339 votes out of more than 50,000 cast.

This year Nix (one of only two blacks to chair House committees) faces another tough fight from challenger William H. Gray III, a man half his age, with an impressive grassroots political organization. Gray. 36, is a pastor of the Brighthope Baptist Church, whose 3,000 members exert a major influence in the political life of the district that Nix represents.

Federal and postal unions have gone all out to support Nix. Officials of AFL-CIO's Committee on Political Education were in Philadelphia this weekend to talk about ways to help Nix overcome his toughest political challenge.

COPE already has authorized a "generous" donation to the Nix campaign. And scores of federal and postal union leader from Washington and Philadelphia, were among the 1,200 who paid $25 a ticket last Thursday night at a fund-raisng dinner for Nix.

Royal Sims national vice president of the Amercian Federation of Government Employes for the Philadelphia region, is backing Nix, and believes he will win. But Sims feels it will be close. The AFGE has nearly 16,000 members in Philadelphia, and about 5,000 in Nix's district. In a very close race, which experts believe this will be, the postal-federal vote could make the difference.

If Nix wins the Mary primary, he certainly will pay careful attention to what AFL-CIO unions - particularly the AFGE - have to say about the President's civil service streamlining plan. Nix has been a strong chairman, and shown that he can get members to show up for or stay away from key committee votes.

All this could be very important to the Carter administration, which wants the "reforms" badly, and to federal employes who will have to live and work under whatever changes Congress and the White House work out in the months ahead.

Right now, though, most people are not looking beyond the May 16 primary that will determine whether Nix will continue to handle the major civil service job changes.

Foreign Service Promotions: The Senate Foreign Relations Committee today is scheduled to reconsider the promotions of 284 Foreign Service officers already cleared by State. Approval of the promotion list - normally routine - was delayed two weeks ago when a former president Association objected to methods used to select candidates for the pay and grade uplifts.

Lars Hydle, currently president of the AFSA, says the candidates were picked in accordance with regular procedures. His only objection is that the list contains so few names. (State has promised a second promotion list later this year)

Insiders expect the Senate unit will approve the 284 names if its resident expert, former FS officer Sen. Clairborne Pell (120D-R. I) gives it the nod.