In a storage room at the A.D.W. Quinn mailing house in Beltsville, stacks of glossy yellow and black brochures are now being labeled and prepared for delivery to the residents of 26 communities in Prince George's County.

The brochures are intended to show what the county has done in recent years to improve its older, inner-Beltway neighborhoods. They also show one of the advantages of being the incumbent in an election year.

Written by the Department of Program Planning and Economic Development and printed at the Central Services print shop, all at taxpayers' expense, the slick leaflets prominently feature the picture, name and thoughts of County Executive Winfield M. Kelly Jr., who is in the midst of a difficult reelection campaign against Republican Lawrence J. Hogan.

Kelly and his programs are portrayed in such flattering terms, in fact, that one of his aides said he would "like to have 20,000 more brochures to hand out on the streets before the (Nov. 7) election."

As it is, more than 23,000 brochures are expected to be mailed before the election, and most are to be sent to working-class communities where Kelly, by his admission, is fairing poorly.

According to the county employes involved in preparing the brochures, entitled "Community Life," it is coincidence that it will appear in mailboxes during the week before election.

"This thing has been in the works before election.

"This thing has been in the works for a long time," said Sal Deleva, an assistant in the program planning department. "I was given the assignment to prepare it last July, but I guess in government it takes a long time to do anything."

The purchases order for the enamel-finished paper on which the brochures was printed, however, indicated a sense of urgency about the project. "This job must be printed and ready for mail out by 11/1/78 per Mr. Gershenow," it read. Louis Gershenow, the central services director responsible for Nov. 1 deadline was artificial.

"There was really no rush," he said. "I just didn't want the buyers to fool around too long."

The "Communuty Life" brochure is the latest in a long line of public-funded mailings and advertisements that the image-conscious Kelly administration has used to political advantage since 1974.

Last year, at the start of the political season, the county spent more than $100,000 on radio ads to promote economic development in Prince George's. The ads featured Kelly's political slogan - "New Quality" - and sounded similar to radio ads that Kelly is now running, at his expense, during the campaign.

Two months ago, when property tax bills were sent out in the county, they were accompanied by a letter from Kelly in which he took credit for the first real tax cut in county history." - a cutback in property tax payers' actual bills, rather than just in the tax rate.

Kelly and his aides are open about their efforts to mix government with partisan politics. "There isn't any difference between the two," said Kelly aide John A. Lally. "Everything an elected official does has a political meaning to it.

This theory was applied on a different level recently when Kelly directed his appointees on the Washington Suburban Sanitary Commission to stall until after the election a decision to hike the rates on water hook-ups for new developments and houses converting from septic tanks to the bi-county water system.

"Sure, that was a political decision," said a Kelly aide. "We were afraid that voters would misinterpret the rate hike and think it applied to all homes. We weren't about to get caught up in explaining that in the middle of an election."