Dart Group Corp., owned by Washington's Haft family, late yesterday offered $30 a share, or about $840 million, for Stop & Shop Co., the Boston-based operator of nearly 300 supermarkets and Bradlees department stores.

Officials of Stop & Shop, which is the nation's ninth-largest supermarket chain, could not be reached for comment late yesterday.

Herbert H. Haft, chairman of Dart, and his son Robert Haft, president of the company, own Crown Books Corp. and Trak Auto Corp. Until 1984, they also owned Dart Drug Corp. Robert Haft said in an interview yesterday that he believed the skills the Hafts have acquired in running those businesses would complement the businesses of Stop & Shop.

"Stop & Shop is now getting into the drug store business, much as Giant and Safeway are here," Haft said. "We could bring some expertise to that part of the business. ... We think it's a generous offer."

The Hafts already own almost 3 percent of Stop & Shop stock and had contacted the company previously to express interest in acquiring more.

In a letter sent yesterday afternoon to Stop & Shop, the Hafts said they were prepared to commit a substantial amount of their own capital to the deal. The letter also said PaineWebber Inc., Dart's financial adviser in the takeover, had issued a letter saying it was confident that it could arrange the remaining financing for the company.

Robert Haft declined to disclose any of the financial details yesterday, saying he wanted to meet with officials of Stop & Shop first. However, Dart Group reportedly has about $400 million in cash available to it now.

Stop & Shop's stock, of which there are 27.9 million outstanding shares, closed yesterday at $26.12, up 12 cents. The Hafts, who began buying stock in the company late last year for about $18 a share after the stock market collapse, now hold less than 3 percent of Stop & Shop stock, or about $15 million worth.

Dart Group applied last week to the Federal Trade Commission for approval to acquire more stock.

Over the past several years, the Hafts have made several unsuccessful bids for other retail chains, including Supermarkets General Corp. and Safeway Stores Inc. Last year, the Hafts failed in a $6.3 billion attempt to take over Dayton Hudson Corp., the nation's sixth-largest department store chain, losing $67 million in the process. When the stock market collapsed in October, Dayton Hudson's stock price dropped far below the price Dart was offering, making its bid too difficult to finance.

The company also recently was reported to be acquiring stock in Federated Department Stores, but, according to Wall Street sources, sold the stock back to Federated for nearly $200 million.

In their letter to Stop & Shop officials, the Hafts said they hope to retain current management if the deal is completed, including some members of the board. The letter also noted Dart Group's intent to keep Stop & Shop's headquarters at its present location.

"We're reaching out to them," said Robert Haft. "We hope we'll have a meeting in a short period."

Stop & Shop had sales of $3.8 billion in its last fiscal year, and a profit of $38.4 million.

While publicly held, Stop & Shop also has a sense of family history. The company was founded by several families, including ancestors of Avram J. Goldberg, the company's chairman, and Carol Goldberg, its president.