D.C. police and union negotiators were scheduled to meet this week with an outside mediator for the first time in an effort to hammer out a contract after 18 months of negotiations.

Resolving the contract is seen as a critical way to ease tension between management and the city's 3,800 officers and to help keep the department focused on fighting crime. The District is on pace to record its lowest annual number of killings in nearly two decades.

Union and police officials would not discuss specifics of the proposed contracts or what has been said in negotiations.

But newsletters and memos distributed by both sides to the rank and file reveal that the department would like the union to accept a 21 percent raise over five years. The union is seeking a three-year contract and is unhappy with how the raises would be phased in.

Both sides are also battling over how the department will handle performance reviews and internal discipline procedures and over whether to return to a 20-year retirement program. An officer hired today must work 25 years to earn full pension benefits.

The first mediation session was scheduled to take place yesterday, and both sides say they are optimistic that their differences can be worked out.

However, recent comments and actions by both sides have cast doubt on some of the optimism.

Last month, after almost a year without a contract, the union declared that the parties were at an impasse.

Moreover, union officials recently filed unfair labor practice complaints that accused D.C. Police Chief Charles H. Ramsey of breaking negotiation rules and improperly appealing directly to officers over a talk-radio show and in a widely circulated memo.

In the letter, Ramsey accused union officials of being "heavy on rhetoric, but noticeably light on facts and details."

Union officials called Ramsey's letter an end run around the union's negotiating team.

"He thinks he's above the law," said Sgt. Gregory I. Greene, chairman of the D.C. Police Labor Committee for the Fraternal Order of Police Lodge 1.

Union officials, who have complained vocally about poor management decisions and low morale, have also expanded their criticism beyond traditional labor issues.

Early this month, the union asked the D.C. inspector general's office to investigate allegations that the department had improperly counted crimes in one of the city's busiest police districts.

Union leaders said they were concerned that the department was intentionally under- representing the city's crime problems.

Ramsey has defended the department's crime statistics and said union officials were raising the issue only because they could not reach agreement on a contract. The inspector general has agreed to conduct an inquiry.

Despite the recent tough talk, union and police officials said the actual negotiations have gone more smoothly than in previous years.

Gary Hankins, a union consultant and former head of the D.C. police labor committee, said talks in 1985 were much more tense.

At one point back then, Hankins recalled, city officials asked union members to check their handguns at the door.

If a mediator fails to help the two sides reach an agreement, the matter will go before an arbitrator, who will present a contract to the D.C. Council for approval.