If the Washington Diplomats are to keep the Cosmos from running away with the NASL's Eastern Division title this season, they'll have to beat the champions in tonight's 8 o'clock rematch at RFK Stadium.

Diplomat officials said yesterday they are expecting between 20,000 and 25,000 fans for the match between the top two teams in the division.

One of the Dips' primary goals this season is to win every home game. The club, 4-0 in RFK thus far, is determined to avenge a controversial 1-0 loss to the Cosmos three weeks ago, in which the Diplomats claim the winning goal should have been nullified by a call of a hand ball.

But Washington (6-3, 47 points) will have to fight the Cosmos with more than inspiration. The Diplomat defense and goalkeeper Jimmy Brown played almost flawlessly in the earlier loss, but the offense spent the afternoon counterattacking.

Tonight, the Dips will face a Cosmos team (8-2, 71 points) that lost two straight, to Chicago and a home exhibition to Belgium's Anderlecht.The Dips think they have a good chance to extend that streak, especially since the Cosmos' starting midfielders, Angelo DiBernardo and Rick Davis, are out with leg injuries and Seninho is doubtful.

Here's what the Dips must do to defeat the Cosmos:

The Dips feel they must score the first goal. The Cosmos scored first last time and played keepaway thereafter. The Dips' offense is not the team's strong point. "We don't want to allow the early goal," said Brown. "We must score first and get control of the game."

The Dips have to attack constantly. They can't afford to try to counterattack. "We have to be aggressive and run more than we did last time," said midfielder Heinz Wirtz. "The more we push them, the more we'll keep them off us," said Ross Jenkins, who will start up front with 22-year-old American Ole Mikkelsen.

The Cosmos outran the Dips on their Giants Stadium rug, but the Dips contend RFK's Bermuda grass will make a big difference in running and dribbling. Some of the players expect a high-scoring game. "We'll score goals against the Cosmos, for sure," said defender Ivan Belfiore. "This won't be any 1-0 game," said Brown.

Washington must get better play from the wing positions. The Dips got none three weeks ago. It's almost impossible for a team to successfully attack the Cosmos up the middle. But the Dips didn't have a true winger until they acquired Willi Kiefer later that week from West Germany. Kiefer won't start but may see a lot of playing time in the second half.

"We absolutely must improve our wing play, whether it's the midfielders taking to the wings or Willi playing a wing," Wirtz said. He is Kiefer's interpreter and may be the midfielder called upon to move up and over when the Dips are attacking.

The Diplomat defenders must play as intensely as they did in the first game. And that starts with centerback Peter Carr, who again will mark Cosmos star Giorgio Chinaglia. Three weeks ago, Carr held Chinaglia scoreless. Carr pulled a muscle in his right leg early this week in practice but says he is all right. "It wouldn't have made a difference anyway," said Carr, "because I kick Chinaglia with my left leg."