In a move likely to intensify the Washington auction market in art works and antiques, a group of dealers and collectors that includes former Secretary of the Navy J. William Middendorf II has made "a substantial investment" in C.G. Sloan Co., the downtown auctioneers.

Sloan's, 715 13th St. NW, is Washington's most ambitious art auctions have been organized by Sloan's competitors, Adams A. Weschler & Sons, 905 E St. NW.

In December 1975, for instance, Weschler's held a four-day million-dollar sale of art works and antiques attended by dealers and collectors from throughout the country. Donald D. Webster and Russell E. Burke, the Chevy Chase dealers and appraisers, helped write the catalog entries and organize that sale. But Webster and Bourke, who have extensive art market connections, are among Middendorfs associates, and they now will work with Sloan's.

"We are most enthusiastic," Webster said yesterday. "Sloan's is a fine old business with grand reputation and a first-rate profit potential." In preparation for a four-day half-million dollar catalog auction, whose 1415 lots will be previewed at Sloan's auction rooms tonight, the floors have been refinished, the walls have been repainted, a 128-page catalog has been printed, and a national advertising campaign is underway. The sale, of bronzes, elephant tusks, jewelry, paintings and other objects, begins Feb. 3.

"We left Weschler's amicably," said Burke."This used to be a one-horse city. It's now is a two-horse town."

Middendorf, a former banker and ambassador to the Netherlands who resigned his Navy post on the day of President Carter's inauguration, is a man of many parts. In 1944 he had a professional baseball tryout with the New York Giants. He writes symphonies and marches, and designs stained-glass windows. He also owns an important art collection.

Last September, a Middendorf Rembrandt, the "Portrait of Juno," was sold anonymously to the oil billionair Dr. Armand Hammer. The price was $3.25 million, a record for a picture by the 17th-century Dutch master.

While serving with the government, Middendorf was often seen at Washington galleries, antique shows and art auctions. His nephew, Chris Middendorf, operates a commercial gallery at 2014 P St. NW, but former Secretary Middendorf is not an associate in that venture.

Washington has long had a thriving market in contemporary art - expensive works by Rauschenberg and Warhol are for sale here. But despite the splendid holdings of Washington's museums, and the sophisticated audience those museums have helped form, the re-sale market here, especially for older works, has been relatively weak.

For a variety of reasons that situation may change. Spurred perhaps by the historicism of the Bicentennial, Americans are increasingly investing in older works of art. The wide-spread enthusiasm for contemporary works, so apparent in the '60s, seems to be diminishing. Concurrently, America has been replacing England as the center of the international market in Old Masters and antiques.

The New York auction market is thriving. During the 1975 season, for the first time, Sotheby Parke Bernet did more business in New York than in its London salesrooms. This spring, perhaps in response to that development, Christie's, Sotheby Parke Bernet's arch-competitors, will attack the American market by opening a pair of New York salesrooms.

"If Sloan's does as well as we expect it to do," said Burke, "we may discourage the London firms from opening in Washington."

Sloan's is planning three catalog sales for this season. Rugs, furniture and art works from "Firenze," the Northwest Washington estate recently sold by Polly Logan, the former Mrs. M. Robert Guggenheim, will be offered there in March.

Among the 1415 lots in the February auction are an 1869 Jasper Cropsey landscape, "Wassawandas in Autumn" (estimated price $25,000-35,000), an 1860 statuette by John Quincy Adams Ward, "The Indian Hunter," which is one of the earliest American neo-classical bronzes to celebrate the Indian ($15,000-30,000), an autographed Hemingway novel ($200-300), and the pen with which President Lincoln "signed the bill giving freedom to all the inhabitants of the territories of the United States" ($1,000-$1,500).

"Sloan's has been in the doldrums," said Burke. "For one thing, the Redevelopment Land Agency owns this building, which makes planning for the future difficult. The auction business requires substantial capital for advertising, catalogs and for aggressively pursuing buyers and consignors. What Sloan's needed - and what it has been given - is a new infustion of money and expertise."