Don't be surprised if you see Marion Barry thumbing through his political manuals, in search of the chapter headed: "The Advantages of Incumbency."

Certainly, the recent campaigning for D.C. mayor hasn't made incumbency seem such a good deal. Patricia Roberts Harris, the former secretary of HUD and HEW, has been harping on Barry's "sorry performance" and his "failed" administration. Council member John Ray, once a Barry supporter, is lambasting the mayor for his inability to get the city's water bills straight. Betty Ann Kane, also a member of the council, has attacked him for his "poor investment practices"--her evidence being a check to the city for nearly $100,000 which remained undeposited from Feb. 18 to the end of April, costing the city hundreds of dollars in interest. Council member Charlene Drew Jarvis has charged him with responsibility for the "loss of business confidence" that is costing the city jobs and revenue.

Advantages of incumbency indeed!

This time four years ago, the two major challengers to Mayor Walter Washington were scarcely mentioning the incumbent. Council chairman Sterling Tucker and at- large council member Barry were running almost exclusively against each other. This time, the challengers are ignoring each other and focusing all their fire on Barry.

As a result, even Barry supporters agree, the incumbent is running scared-- though perhaps not as scared as the early evidence might suggest.

If you think of the Democratic electorate as consisting of pro-Barry and anti- Barry blocs, it isn't exactly devastating to Barry's reelection hopes that the anti- Barry vote is split seven ways--among Harris, the three council challengers and three less well-known contenders: Morris Harper, Denis Sobin and Richard Jackson.

The thing to keep in mind is that ousting the incumbent is not a team sport. If you are the challenger, your purpose isn't merely to see that Barry loses, but to see that he loses to you. Barry, no doubt, would like to see this crowded field stick around until the Sept. 14 primary--even if the majority of the vote goes against him. He doesn't need a majority--only a plurality.

He'll have no such luck, of course. An Associated Press/WRC-TV poll released this week shows that Pat Harris has broken out of the pack--even to the point of establishing a lead over Barry, 39 percent to 28 percent. No one else finished in double figures. What that means (apart from the obvious implications for political fund raising) is that the other challengers will no longer be able to take their shots solely at Barry. They'll have to target Harris as well, which means that they'll be doing some of Barry's dirty work for him. What happens after that will depend on how vulnerable Harris is to direct attack, and how well she can handle being on the defensive.

It depends, too, on how skillfully Barry is able to use the news media: to campaign without seeming to campaign. You can get only so far with the occasional screwed-up water bill or the random undeposited check if you have no power to deliver anything.

Challengers can complain about crime; the incumbent can redeploy the police force. The challenger who talks about potholes or trash collection or business failures is seen as playing politics. The incumbent who repairs streets, cleans up alleys or announces that a new firm has come to town is seen as merely doing his job.

That, along with the power to reward friends and punish enemies, is the real advantage of incumbency. It doesn't make you invincible, as the recent poll suggests. But it surely doesn't hurt.