The Office of Personnel Management is expected to withdraw today its long-pending plan to revamp the seniority-dominated system used by the government to decide which federal employes receive longevity pay raises and which ones are laid off.

The rules change, which would have devalued seniority as the primary factor in pay raises and reduction-in-force actions, was scheduled to go into effect Monday.

Any changes in the policy governing within-grade raises would have a major impact on the federal work force. Nearly 100,000 white-collar civil servants in the Washington-Baltimore area become eligible for the longevity step increases each year.

In addition, the proposed changes would have revamped the government's last-hired, first-fired layoff policy. During the first 2 1/2 years of the Reagan administration, 2,900 federal employes here were fired for economy reasons, and many more were demoted or downgraded because of reductions in force (RIFs). Most of those fired were those with the least seniority in their agency.

Congressional sources say that "very delicate" negotiations between the OPM and Rep. Steny Hoyer (D-Md.) resulted in OPM's decision not to make any changes without consulting Congress. Acting OPM Director Loretta Cornelius, anxious to reduce four years of tension that have built up between Congress and the agency, has apparently agreed to the withdrawal. OPM officials said they cannot comment, but the action is expected to be published today in the Federal Register.

Cornelius is in charge of OPM until Congress acts on the nomination of Constance Horner, an official of the Office of Management and Budget, to head the agency. Cornelius moved up to acting director when Donald Devine's four-year term as director expired.

Dropping the controversial rules change, which OPM has pushed for four years, would be a major departure from Devine's policies. He argued that the current system gives no authority to managers or incentive to workers because the step increases are almost automatic. He said the seniority-based RIF system forced agencies to fire their brightest younger employes.

Congress has twice blocked the OPM rules changes, which would have made it tougher for employes, who now get virtually automatic 3 percent raises every 12 to 36 months, to qualify for step raises.

It also banned OPM from assigning seniority points to employes based on their annual performance ratings. Such a system would have made it possible for a worker with relatively short service who had three outstanding performance ratings to rank higher on job retention lists than a more senior worker with a mediocre track record.

The most recent ban on the proposed OPM rules change expires at midnight Sunday. Before he resigned from the OPM, Devine said that he would implement those rules as soon as the ban expired unless Congress acted to extend it. Legislation -- in the form of a rider to an appropriation bill -- is pending to extend the ban until October, but its quick passage is considered unlikely.

Congressional sources still expect OPM to push for some kind of pay-for-performance system. But it will not be proposed again until congressional objections have been met, particularly in the area of linking performance ratings to job security.