"I think you have to rewrite that old expression," Rep. Margaret Heckler (R-Mass.) told the gathering last night in the lofty reception room of the Organization of American States building. "Tonight you have to say, Never underestimate the power of the Congresswomen's Caucus.'"

From the looks of the impressive crowd that came to celebrate the second anniversary of the caucus, she had a point, Joan Mondale, wife of the vice president, Sen. Edward M. Kennedy (D-Mass.), House minority leader John Rhodes, (R-Ariz.), House majority leader Jim Wright (D-Tex.), and Mayor Marion Barry and his wife Effi all swept in to the reception, if only-for brief stays.

"The most important reason why I'm here," Rhodes said to the group, "is because Peggy Heckler told me to come. So I came."

Sen. Kennedy briskly hopped to the podium to say, "I knew I'd be among friends when I looked at the program and saw that the American Medical Association was one of the patrons."

The party was also a fund-raiser for the new Women's Research and Education Institute, the research arm for the non-partisan caucus.

And then there was the receiving line of caucus members themselves-Lindy Boggs (D-La.), Shirley Chisholm (D-N.Y.), Cardiss Collins (D-I11.), Barbara Mikulshki (D-Md.), Mary Rose Oakar (D-Ohio), caucus co-chair Elizabeth Holtzman (D-N.Y.), Patricia Schroeder (D-Col.), Gladys Spellman (D-Md.), and new representatives Geraldine Ferraro (D-N.Y.), and Beverly Byron (D-Md.).

Although all 16 women members of the House and the one woman of the Senate (Nancy Kassebaum, a Republican from Kansas) are all members of the caucus, they were not all there last night.

But taking up slack in the receiving line were HUD Secretary Patricia Roberts Harris and Treasury Secretary Michael Blumenthal. "This is an honor," said Blumenthal. "I feel right at home."

Also among the guests was Bella Abzug-looking splendid in a straw hat and black suit-who was greeted with great fanfare and affection by all including Tim Weiss (D-N.Y.), Abzug's successor in Congress, who embraced her warmly.

And Abzug's successor to the chair of the President's Advisory Committee for Women, Lynda Robb, also greeted Abzug warmly, before engaging her in a conversation about how to take care of committee business. ("Did you write any letters or did you just call people"? Robb asked.)

Heckler, a co-chair of the caucus, mentioned such caucus accomplishments as passage of the Rape Victim's Privacy Act and passage of the Displaced Homemakers Assistance Act. She said later the most lasting contribution was probably the passage of a bill authorizing veterans benefits for the women who ferried heavy aircraft in World War II (a group called Women As Service Patrol). "It was a major breakthrough," Heckler said. "It showed our strength - it showed 15 women could intimidate 100-plus men."

Pat Schroeder, there will her 8-year-old daughter, Jamie, surveyed the crowd. "Obviously I wish we could do more. But it's a problem of numbers. Numbers speak in Congress. But considering them, I think we've done phenomenally." CAPTION: Picture 1, Wiley Branton, Jack Greenberg and Vi Curtis Hinton at party commemorating Brown decision, by Margaret Thomas-The Washington Post.; Picture 2, Rep. Augustus Hawkins, Coretta Scott King; by Joe Heiberger-The Washington Post; Picture 3, Edward Kennedy and Mrs. Peter Jay with Elizabeth Drew; by Harry Naltchayan; Picture 4, From left, Patricia Harris, Elizabeth Holtzman, Michael Blumenthal and Margaret Heckler; by Fred Sweets-The Washington Post.