A move to deny Rep. Robert N.C. Nix (D.Pa.) the chairman of the House Post Office and Civil Service Committee has quietly died.

House sources said the move, initiated by younger members of the 28 member committee, has been laid to rest for two reasons. One is that Nix, an 18-year veteran of the House, is black and any concerted move to block his getting a chairmanship would be met by charges of racism, the sources said.

Said one congressman familiar with the now defunct skirmish: "Nix - he's a decent man. But the feeling around here is that he just doesn't do anything, and someone stronger is needed to grapple with the problems of the committee. If he were white, there probably (still) would be a quiet move afoot to unhorse him . . . But nobody around here wants to be accused of being a racist."

Second, the sources said the anti-Nix move died because there is lingering belief that - despite its potential importance in the administration of President-elect Jimmy Carter as an instrument for reshaping the federal bureaucracy - the Post Office and Civil Service Committee chair is not worth fighting over.

"It's a bad news committee," said an aide to Rep. James M. Hanley (D-N.Y.), who had been urged to challenge Nix for the chairmanship.

"If you're out there campaigning, it doesn't help matters any if your opponent can say that you're responsible for the terrible way the Post Office operates, or that you're responsible for the 14-cent stamp.

"Fighting over who's going to give all that bad news to people is not exactly something that congressmen want to get into," the Hanley aide said.

Nix's prospective elevation to the chairmanship of the committee was made possible by Rep. Morris K. Udall's decision to take over the Interior Committee. Udall, an Arizona Democrat, had been next to line to chair the post office committee.

But the election of Rep. Jim Wright (D-Tex) as House majority leader changed all that by opening up two other committee chairmanships - Public Works and Interior. Udall said he decided to seek the Interior Committee post because it is more important to his state.

That decision disappointed many members of the post office committee who had hoped that Udall would use his national reputation to give the committee new stature and direction.

Sources said the younger committee members, as well as some members of the committee's policy staff, were particularly upset upon learning that Nix, was next in the after Udall.

The younger members charged that Nix's age, which is officially listed as 68 but which opponents say is greater, and his "recorf of inactivity" in the House made him unfit for the chairmanship, the sources said. Nix's opponents also claimed, as was alleged last month in a Jack Anderson column, that Nix drinks excessively.

Nix could not be reached for commment, despite repeated calls to his Washington office and his district office in Philadelphia.However, in the Jack Anderson column, he denied allegations about drinking. "I've never had any problem with alcohol in my life," Anderson quoted Nix as saying.

The opposition to Nix generated the drive to back Hanley for the committee chairmanship, the sources said. And, said the sources, the pro-Haley drive immediately met opposition from members of the Congressional Black Causs - especially from Rep. William Clay (D-Mo.).

"Clay worked it very cleverly," one committee member said. "He took it on almost by himself. He called a meeting of the "Democratic" committee members, including Nix, and said that the committee would fare well under Nix because he would give the members wide latitude. He also threatened that he and other members of the caucus would make a big fuss if Nix didn't get the chairmanship," the committee member said.

Clay also is a member of the post office committee.

Al Johnson, a spokesman for the Missouri congressman, confirmed that Clay was largely responsible for stopping the anti-Nix move. He said that the meeting of the Democratic committee members took place Dec. 20.

But he said the meeting was called by Nix.

"When Bob Nix decided that he wanted the position, he asked all of the Democratic committee members to come and sit down with him to share his concern," Johnson said. "He listened to them and some of their misgivings and they listened to him. It worked out okay," he said.

Johnson said he doubts that the Hanley backers gave Hanley their wholehearted support.

"I don't think anyone really supported Hanley," he said. "They were just opponents of Nix. Besides, Hanley wouldn't get any political benefit by serving as chairman of this committee," Johnson said.

Asked what, if any political benefit Nix would receive by assuming the post office committee chairmanship, Johnson said: "There is only one black who is chairman of a full congressional committee. That's (Rep. Charles C.) Diggs. Nix would make the second black chairman of a full committee."

Diggs, a Michigan Democrat, is chairman of the District of Columbia Committee.

According to a Ralph Nader Congress Project profile of Nix, the congressman entered politics in 1932, at age 30, as a ward committeeman for the Philadelphia Democratic Party. He has quietly represented his Philadelphia constituents in Congress since 1958.

Nix's opponents point to his long political career as proof that he is at least six years older than his stated age of 68.