The Office of Personnel Management has rejected a request from the powerful Senate Appropriations Committee that OPM delay until Oct. 1 making changes in rules covering the pay, promotions and firing procedures of the government's 1.4 million white-collar workers.

OPM says it still plans to put the revised work rules into effect starting in mid-August.

Both the Senate and House are working on legislation relating to the OPM work rules changes that would have an effect on 300,000 Washington area civil servants. But neither has much chance of passage before Congress recesses next week.

The Senate plan, by Assistant Majority Leader Ted Stevens (R-Alaska) would limit OPM's pay-for-performance program to a 3 1/2-year test program, covering 150,000 workers. Unions would be able to negotiate terms of the pay and layoff rules.

The House proposal, by Rep. Steny Hoyer (D-Md.) would bar OPM from making any of the changes.

But Congress is due to recess next week and not return until mid-September. And if OPM sticks to its timetable, the rules change would go into effect before then.

Sen. Charles McC. Mathias (R-Md.) plans to introduce an amendment to the conference report of the supplemental appropriations bill that would block the OPM changes. Such an amendment to a conference report is rare. If the Senate should clear the report with the Mathias amendment, the package would have to go back to the House. That could delay the entire supplemental package--which includes money to operate the government for the next three months. Congressional leaders would like to avoid such a hassle this close to the recess.

To that end, four key members of the Appropriations Committee yesterday sent a hand-delivered letter to White House chief of staff James A. Baker III. The letter, asking the White House for more breathing space on the OPM regulations, was signed by Chairman Mark Hatfield (R-Ore.), James Abdnor (R-S.D.) chairman of the Treasury subcommittee and ranking minority member Dennis DeConcini (D-Ariz.) and John C. Stennis of Mississippi, the ranking Democrat on the committee that gives out--or withholds--money to federal agencies.

Their letter said the Mathias amendment probably would be approved, in which case the supplemental package would go back to the House, "delaying the passage of this urgently needed supplemental bill.

"We may be able to avoid consideration of such an amendment if the administration will agree to delay publication of the final OPM regulations until Oct. 1," the letter said.

The White House response was prompt: It said that OPM would not agree to any delay.

Top OPM officials were on Capitol Hill yesterday trying to line up support to kill the Mathias amendment, and to block any House action that would stop their regulations.

A committee staffer said yesterday that some members are "furious with the OPM" for the turndown.

OPM has clearly decided to play hardball. But the team it is up against (the Senate Appropriations Committee) doesn't lose many when it comes to getting what it wants (one way or another) from executive branch agencies such as OPM, which doesn't know, or care, about this town's power pecking order.