Just before it left town, Congress slapped a two-month hold on Reagan administration plans to devalue seniority and make performance the determining factor in pay raises, promotions and layoffs for the government's 1.4 million white collar workers.

If Congress had not acted, the Office of Personnel Management had planned to put new work rules--called the Performance Based Incentive System--into effect starting today for the 360,000 white collar workers in the Washington/Baltimore area.

Under the PBIS system, now on hold until Oct. 15, an employe's immunity from being fired during a reduction-in-force (RIF) would be based on performance rather than seniority.

Since the Reagan administration took office, several thousand Washington area feds have been RIFfed, most of them employes with the least seniority.

In addition, the OPM-designed layoff rules change within-grade pay raises, now virtually automatic, to make them tougher to get. Under OPM's proposal, those raises, which are now based almost entirely on time in grade, would be withheld from workers who didn"t get good marks from their bosses.

Employes at the upper end of their particular grade (those with the longest service) would be required to get even higher performance ratings from their bosses to get the raises.

Federal employe unions balked at the OPM proposals, even after they were modified slightly by the personnel agency.

Congress, anxious to take its midsummer vacation, tried to get OPM to agree to delay making any rules change. But OPM said it would make the changes starting today unless Congress told it, in writing, not to do it.

Just before the recess, the Senate and House overwhelmingly approved language blocking any rules change until mid-October. It added that language to the transportation bill, which is considered veto-proof.

President Reagan has until midnight Wednesday to sign the bill. OPM officials said he would sign the measure, even with the language calling for a temporary delay in the rules change.

"If the ban had been permanment instead of a two-month delay, that would have been another ball game," an OPM official said yesterday.

Assuming the president signs the transportation bill, the rules changes will not be made until Oct. 15. Federal unions think that will give them time to kill or gut the proposals. The OPM is counting on Congress' getting involved in more important matters--like money bills to run the government--and losing interest in continuing to fight the rules changes.