It was probably the first time that the Capitol Building, seat of the legislature for the greatest power on earth, has been the site of a "news conference" to announce a retail company branch.

And some of the legislators took time out from their consideration of a Panama Canal treaty, energy policy, taxation and airline industry deregulation to show up.

Sen. Mark O. Hatfield (R-Ore.) was there. And so was Sen. S. L. Hayakawa (R-Calif.). And Rep. Newton I. Steers (R-Md.).

The event was held yesterday morning by the prestigious I. Magnin specialty chain of San Frncisco to announce "formally and officially" what had been reported widely before (for example, in a Magnin catalogue sent to 20,000 existing area charge account customers).

According to the official hand-out, the rationale for a Capitol locale was to brief members of Congress from the 23 districts and five states in which Magnin has 23 stores about the company's first East Coast branch.

To be located in White Flint Mall on Rockville Pike, the 81,000-square-foot, two-evel store represents an investment of $10 million and is scheduled to open on Aug. 11. Sales of more than $8 million are expected in the first year.

Rep. Steers was particularly interested in the arrival of Magnin to his congressional district along with 150 new jobs in a store under the direction of Magnin Regional Vice President Jay Litch, whom Steers congratulated on his choice of a home in Montgomery County (Litch is moving here from Chicago).

But Steers also noted that the 10-year-old San Francisco firm, known in particular for its women's fashions and personal service, would be joining such other newcomers to his congressional district as Bloomingdale's and Neiman-Marcus -- apparently annexing the small D.C. parcel where Neiman is located into Montgomery County.

Hatfield read his statement about the arrival of Magnin in metropolitan Washington just as advance press releases from Washington publicist Patty Cavin said he would. It's "very exciting news," he said.

As might be expected, it was Hayakawa who did something different. The noted linguist wasn't there for the start of the news conference but sent along his prepared remarks on a tape recording while he participated in a congressional hearing. "It makes sense" for Magnin to come east, he said on tape, "as that's where the tax dollars flow." And, within a few moments, he showed up in person.

Ross Anderson, the board chairman of I. Magnin & Co., conceded that he knew of no other press conference to announce a new branch store that had attracted U.S. senators to the Vandenberg Room of the Capitol. And Hatfield noted that he wasn't there to give Magnin a "testimonial," but just to share "our Western institutions" with people here.

It was a coup of sorts. Such local retail giant and future Magnin competitors as Garfinckel's, Woodward & Lothrop and the Hecht Co. never thought of announcing expansion plans at the U.S. Capitol. Of course, these Washington-based firms have no voting representatives in Congress, either.

It all started when Bloomingdale's, like Magnin a division of the Federated Department Stores Co. and a White Flint tenant, announced its final Washington store plans at a sedate Madison Hotel gathering, following several years of zoning fights.

To top that, Neiman-Marcus of Dallas hosted a reception for the press in a Kennedy Center anteroom. Magnin had to do something different.

All of which raises these questions: When the few remaining big-name stores not here yet decide on locations for their stores, will they be able to make an announcement at the White House?