An advertising tabloid promoting Prince George's County that was scheduled to run in The Washington Post five days before the Sept. 12 primary election has been postponed for one week because of complaints that the supplement would unfairly boost the political campaign of County Executive Winfield M. Kelly Jr.

Several Republican and independent Democratic candidates charged that the 35- to 40-page advertisement would benefit Kelly's reelection campaign by featuring the same "New Quality" theme that the image-conscious county executive has attempted to make the slogan of both his administration and his political career.

The critics included Lawrence J. Hogan, a Republican candidate for county executive, who called the proposed tabloid a "phony political public relations gimmick," and Tim Maloney, an independent Democrat running for the Maryland House of Delegates, who said it was "misleading and partisan in the extreme."

The decision to postpone the supplement's distribution until after the Sept. 12 primary was made yesterday when The Post's advertising department became aware of the controversy. The county groups agreed to the postponement. "It's no skin off our backs," said one Kelly associate. "It was your (The Post's) idea in the first place."

The supplement, nearly identical to one that ran last April in The Washington Star, is being sponsored jointly by the county's economic development committee and Citizens Working for a New Quality, a nonprofit group headed by Kelly's public affairs officer, Richard Scott. Its cost - an estimated $50,000 - is to be borne solely by county businessmen who advertise in the tabloid.

Robert McCormick, vice president for advertising at The Post, said the newspaper's advertising division contacted Scott shortly after The Star's supplement appeared and asked whether The Post could develop a similar package. He said the Sept. 7 distribution date was chosen "innocently" - without knowledge of its possible political ramifications.

"They (the county groups) didn't want it to run too close to The Star's and they wanted it to come out during the back-to-school season," said McCormick. "I really don't think there was anything political about it."

McCormick said the supplement was expected to be approximately 36 pages, with 28 pages containing business advertisements and eight pages reserved for "editorial content" prepared by the county groups and edited by the newspaper's advertising department.

In a pamphlet distributed to potential advertisers, Scott's "New Quality" group said the "editorial content" will deal with the "considerable progress" the county has made in economic development, arts, public safety, education and government. "All are relevant to the quality of our daily life," Scott wrote, "and will be treated in a positive manner to encourage the future growth of our county."

The cover of The Post's supplement is to feature the county seal with the words: "Prince George's County: A New Quality of Life."

"New Quality" has been Kelly's political theme since his 1974 election campaign against then-incumbent county executive William Gullett. He has used it in campaign brochures, banners, county pamphlets, magazine advertisements and radio spots - sometimes at private expense, other times with county funds.

Kelly and his political aides say that new quality is "a state of mind" that they are trying to develop in the county. In specific terms, they say it is represented by encouraging "clean" industries and corporate offices to locate in Prince George's while discouraging the construction of more garden apartments and low-income housing projects.

The opponents of Kelly's Democratic administration say the new quality campaign is one of image, not substance.