Under pressure from the White House and others, Rep. Morris K. Udall (D-Ariz.) has agreed to take on the role of mediator among the factions threatening to stall the president's civil service overhaul bill, Udall said through an aide yesterday.

As vice chairman of the House Civil Service Committee, Udall will fill a vacuum left by the committee's chairman, Robert N.C. Nix (D-Pa.), who has been spending most of his time lately campaigning for reelection in a tough primary race in Philadelphia.

Parties on all sides stressed that Nix had concurred in Udall's stepping in to replace him as the congressional custodian of this political hot potato.

Udall's involvement is widely percieved as a big boost for the president's bill, because the congressman has long supported the cause of government reform.

It is also one more sign that there is a good chance the committee will report out some kind of a civil service bill' for action this session, according to a number of Capitol Hill sources. They attribute the growing momintum primarily to the president's recent personal involvement in pushing the legislation.

in addition to a series of personal contacts with various members and staff working on the bill, Udall has agreed to chair two hearings on the subject on May 12 and 15, and two markup session on May 22 and 23.

Rather than offering his own substitute bill, Udall is working with committee members and staff, as well as the White House, to rewrite the present bill so that it will be acceptable to all sides, according to a Udall aide.

Several Hill sources indicated that concern has been voiced within the committee that the bill would be reported out with the support of all eight Republicans, but only one or two Democrats. As one put it, that "would make it tough sledding when it got to the full House."

Other sources expressed a concern that, given the issue's broad appeal in an election year, the House will pass any bill that makes it to the floor, as long as it is called civil service reform.

"If the (civil service) committee doesn't clear up these ambiguities in the bill, nobody will," said an aide to Rep. Patricia Schroeder (D-Colo.), who chairs a key civil service subcommittee.

Udall in the past had resisted White House efforts to get him more actively involved in the legislation because "he's been hard pressed with other business," an aide said.