Rep. Edward Derwinski (R-Ill.) accused Democrats on the House Post Office and Civil Service Committee yesterday of a "sellout" to federal employes' unions that "could kill" President Carter's civil service reorganization bill.

Derwinski, the committee's ranking Republican, did not withdraw crucial Republican support for the bill, as some of the bill's advocates had feared he might. But he warned the administration that any further concessions to labor might tip the balance.

Derwinski's statement followed a successful maneuver by committee Democrats Thrusday to attach to the bill an amendment that would revise the Hatch Act and enable federal workers to take active roles in partisan politics.

Derwinski said he may try again to get the committee to drop the Hatch Act changes. The committee has already voted twice on the amendment, defeating it narrowly the first time.

Revising the Hatch Act has been a top priority of organized labor. Such a bill passed the House last year with the backing of the Carter administration, but has been stalled in the Senate.

Some supporters of the Hatch Act changes, including the Carter forces and some union reporesentatives, had argued that that issue could not be linked to civil service reorganization without jeopardizing the whole package.

Derwinski stated that "Hatch Act repeal has no chance in the Senate and that the committee Democrats know this. Do they then wish to kill the bill by attaching this political grab of the federal civil service, or do they wish to force the entire package through the House under the guise of civil service reform?"

Invoking the "spirit of Proposition 13," Derwinski added that in either case, "the American public will have been betrayed."

A spokesman for organized labor's Democratic allies on the committee shrugged off Derwinski's remarks, calling them "a lot of rhetoric" from someone who opposes the Hatch Act changes.

Civil Service Commission Chairman Alan Campbell, the administration is "delighted" that Derwinski still supports the bill and that he thinks "the Hatch matter can be worked out satisfactorily."