Rosalynn Carter, the first President's wife in the last 25 years to bypass the congressional fraternity on the way to the White House, was warmly received yesterday by the spouses of that very club.

And even though Mrs. Carter was the outsider among the insiders at the annual luncheon for the First Lady given by the Ladies of the Senate, the private gathering attracted the largest audience in the last decade and wo high marks for Mrs. Carter.

In the receiving lint Mrs. Carter displayed her knack of remembering the places of last encounters. All the wives were wearing enormous identification tags aroung their necks, so she didn't have to struggle with names. "She's marvelous and completely relaxed," said Marchelle Leahy; the wife of the Vermont Democrat.

At the lunch, the tone followed both the social protocol of skipping unpleasant and political conversation, and the casual style the Carters have encouraged.

Out were the plain white tablecloths; in were brightly-colored cloths converted by the wives from sheets called "Saturday's Flower." Out were the formal, engraved menus with the Senate seal; in were menus of orange construction paper with flowers cut from the sheet remnants, assembled by Rita Hollings, wife of the South Carolina Democrat.

Mrs. Carter, wearing a pink wool dress with a full, fringed shawl, arrived promptly at noon and was greeted by Joan Mondale, the president of the group, who was wearing her Red Cross uniform. One congressional wife noted that more uniforms were worn this year. "Perhaps the bitter feelings over the Vietnam War are disappearing," she said. Each Tuesday this group - wives of past and present senators, wives of former Vice-President and wives of Cabinet members - sew, knit and crochet for charities, a practice that started in 1917.

After Mrs. Carter arrived, the giant wood doors of the Caucus Room of the Old Senate Office Building were closed. The room was quickly decorated with flowers and plants borrowed from the Botanical Gardens. The luncheon is so snug an affair that even members of the Secret Service leave the room. A steadfast rule is ladies only, so the women in the Carter Cabinet, not their husbands, were invited, and Juanita Kreps, the Secretary of commerce, attended. Once a member, always a member is another tradition. So the disgraced and divorced are included.

For two hours Mrs. Carter, who sat between Joan Mondale and Lois Ribicoff, chairman of the luncheon, discussed with her table companions her mental health projects, Washington, children, and the menu, selected by Teresa Heinz, wife of the Pennsylvania Republican senator.

"Energy was not mentioned once," said Mrs. Ribicoff, whose husband has been jostling with the Carter administration on energy and other issues. Helen Jackson, wife of the Washington state senator, added, "we want her to feel welcome, we're pleased to have her and entertain her. It's not a day for politics."

Instead of political opinions, the First Ladies are given gifts. Once it was a tree for Pat Nixon, another time a written history of the group for Lady Bird Johnson, another year a quilt and a dance recital by Edward Villella for Betty Ford.

Yesterday Mrs. Carter received a donation of $100 for the Mental Health Association. "It's already spent," she said, explaining that the funds would be used to train volunteers.

Just before the luncheon ended, the U.S. Navy Sea Chanters, introducing their recently-added quartet, the New Helmsman, broke into a few bars of "Georgia on My Mind," "Blow the Man Down," "Rubber Ducky" and "Bye Bye Blues." On her way out Mrs. Carter picked up a cassette of the Chanters.