Members of the House Post Office and Civil Service Committee expect to move forward on President Carter's bill to restructure the civil service system despite the defeat Tuesday of committee chairman Robert N.C. Nix (D-Pa.), according to committee sources.

The defeat of the first black congressman from Philadelphia introduces "one more question mark" into a complex equation and has left some on the committee 'shaken," the sources said. But they do not believe Nix, known to be an "organization man," will use his power as chairman to sabotage the president's bill by refusing to schedule meetings or employing other similar tactics.

Similar pride in their hometown of Baltimore, where ground has been broken for a harbor-front convention center resulted in the failure of two other liberal Democrats, Barbara A. Mikulski and Parren J. Mitchell, to support the proposed District convention center.

Mitchell, head of the House Black Caucus and a good friend of D.C. Del. Walter E. Fauntroy, voted "present," a practice known in legislative parlance as "taking a walk."

"I didn't want to hurt Washington," Mitchell said, "but have strong feelings that two convention centers are not needed in what is more and more becoming the twin cities of Washington and Baltimore."

A Mikulski aide said "Barbara's very pro-Baltimore, protourism," in explaining her vote to table an amendment that added convention center funds to the budget.

The only member of the Maryland delegation who voted in favor of the center was GOP Rep. Newton I. Steers of Montgomery County, who said the center would "help the area's economy in the long run" by attracting major conventions.

In a move to expedite committee action on the bill, now set for next week, the White House with Nix's blessing had already persuaded committee vice chairman Morris K. Udall (D-Ariz.) to step in as a mediator and fill the leadership vacuum left by the campaigning Nix.

Well-liked, witting and a former political foe of Carter's, Udall has been coordinating a painful and tedious compromise effort between the clashing factions whose disagreements had earlier threatened to stall the bill.

In another development yesterday, White House officials agreed to allow interested members of Congress to see a controversial report whose findings had threatened to cloud the civil service revision issue. it was made available to members with the understanding that they would not make its contents public.

The report, prepared by the Carter transition staff contains unsubstantiated but damaging charges against career civil service servants who may have violated civil service laws during the Nixon administration, according to administration officials who had resisted pressure to release it.

Nix, a 20-year veteran in Congress, had agreed initially to navigate the bill forward on the president's behalf. But their alliance was at best fitful.

Even as Carter was gearing up a major effort to improve his administration's troubled relations with Congress, his own Cabinet-level aide, Ambassador to the United Nation Andrew Young, was in Philadelphia making campaign appearances with Nix's opponent in the primary.

Nix at that time warned that if he was defeated, the president would have trouble getting action on his bill from a committee with a lame duck chairman.

Now that his defeat is a reality, however, and he "no longer needs to play hardball with the White House," committee sources said. They said they believe he will not go back on his original deal with the president, however.

Nix might have blamed the president and Young for his defeat had he lost by a slim margin, one sources said, "but the defeat was too overwhelming . . . He got his tail whipped."

Said one committee aide, "There are plenty of people on the committee who still want to deal with the White House, who will have to live with those folks after Nix is gone."

The question now is whether the president will recognize his bill once the committee get through with it. With both Senate and House committee scheduled to take action on the bill in the next week or two, pressures to reshape it drastically have heated up.

President Carter has termed the bill the "centerpiece" of his plan - which was a major campaign pledge - to reorganize the government and make it more efficient. He and his top managers maintain that the bill is necessary to give them the flexibility and control they need in order to carry out government programs effectively.

The plan would streamline federal procedures for hiring, firing and promoting federal workers, set up new mechanisms to protect employes from certain kinds of abuses at the hands of bosses, attempt to tie pay raises to performance instead of seniority and allow top managers to trade some job security for a crack at higher pay for superior performance, according to its supporters.

It also would expand the power of federal employes' unions - though not as much as they would like - and would curtail the lifetime advantages in hiring and job retention now afforded nondisabled veterans. Those advantages, according to Carter, add to the difficulties of hiring and promoting women and minorities.

It was only through a belated White House lobbying bilitz that Carter was able, a few weeks ago, to rescue the bill from premature burial at the hands of its opponents, primarily federal employe unions and the powerful veterans lobby, according to committee sources.

That White House lobbying effort, for instance, had President Carter telephoning Rep. Herbert Harris (D-Va.) to chat about civil service reform at the same time a crucial Panama Canal treaties vote was taking place in the Senate, Harris recalls with a smile.

The pivotal debate in the committee now - over federal labor-management relations - is being pressed by three congressment who champion the interests of organized labor: William Clay (D-Mo.), William Ford (D-Mich.) and Stephen J. Solarz (D-N.Y.).

Some committee sources have expressed concern that, given the issue's broad appeal nationally, the House might pass any bill that makes it to the floor, as long as it is labeled "civil service reform." They feel it is up to the committee, which has traditionally tended to favor federal employe interests, to iron out the potential danger spots, or "nobody else will."