In Jimmy Carter's inner circle of White House assistants, Margaret "Midge" Costanza is the only ethnic, the only Catholic and the only woman.

She's the nice Italian-American girl, who didn't get married, went to work in the office of a cement-construction company, and ended up working with six good ole boys and the President of the United States. And she loves it, wisecracking her way through the day, meeting and talking with groups as various as gays and Indians, and vegetarians, whose Food Day buffet at 1600 Pennsylvania had the cattlemen up in arms.

Costanza has all the gregariousness of her job as liaison to organized America, President Carter's "window to the nation." She's a hugger and kisser who has her picture taken squeezing the White House guards like Charmin tissue. When President Carter introduced her to congressional members she returned his lengthly hug and quipped, "Now you know what I do at the White House." When she runs into an old Democratic friend from the hustings in Nyack, N.Y., she punches him in the paunch, grabs him round the neck and accuses him of lusting after her. He loves it.

Nonetheless, Costanza back in her White House office, a room that is literally a mess of papers - on the desk, under the desk, on the coffee table, stacked and piled and shoved into the crannies and nooks of the room - can telepathically recall what is in each mountain. She can tell you what obscure paper is where and then pull out the exact sheet she referred to.

Beyond the personal magnetism, beyond the comedienne's mien, Costanza is a near-compulsive about her job, working 20-hour days stoked by cigarettes and coffee.

At 44, she has come a long way from Rochester, where she grew up as one of four children of a Sicilian sausage company owner and his wife. "I'm just one of the links," she is fond of saying, explaining that her family was not poor, but merely that "I didn't grow up in wealth, but we never starved. I went to school and had opportunities."

Costanza does, however, worry that she never got to college. She feels that professional administrator types" will find her lacking, even though she has had long years in Democratic state politics.

She began her pilotical life at 18 when she asked a Republican National Comitteewoman how she could get involved and was told to go to work at the ward level.

She got into politics, but always as a bit of maverick. Recalls a Rochester reporter: "She set up an office for Robert Kennedy in 1968 when the party regulars supported Humphrey. It was a shoestring operation and she did everything from washing the walls to cleaning the floors to answering telephones and canvassing the voters. She's incredibly competitive, especially against men.

"But she's not a phony, nor is she personally ambitious. I think when she led the ticket when she campaigned for the city council she was surprised as anybody to find herself in that position."

Before Costanza ran for city council, ending up as vice-mayor - she took a 1974 shot at the House post of Barber Conable. Though she lost, Jimmy Carter was National Democratic Committee chairman at the time. After reading her positions on various issues; he gave her a call, told her they were compatible and asked if she wanted his help in her campaign. The friendship lasted and when Carter campaigned for President, she was co-chairman of his New York campaign.

"If she like you, you don't need another friend in the world," says friend Carol Clifford. "She's a female Rocky. No matter what the decision, she's going to go the distance."

A day with Costanza is like being with a whirlwind, even if she tells you it's a slow day. She makes her own hotel and airline reservations, changing her ticket when she realizes she can get excursion fare. It's New York for the day, a stop in Nyack at the Rockland County Democratic National Committee luncheon and then a late-evening presentation at the Queens Congress of Italian-American Organizations (CIAO). She's hoping to catch up in the interim with Bella Abzug and New York state Sen. Carol Bellamy.

On the plane, she's an odd combination of aggressive friendliness and shyness. She's apologetic to the stewardess has to come and tell her to put out her cigarette. One understands how she ended up, on a friend's advice, living in a $1,170-a-month apartment when she makes $2,400 a month. (She asked the management to take out the gold faucets and is now thinking about getting a roommate to share the expense.)

In New York, she gets picked up by friends and driven to Nyack where she is kissed by people who tell her she shouldn't smoke so much and asked why she hasn't returned their calls or answered their letters. She explains that she gets 350 letters a day and thousands of calls a week.

She gives a short speech to the Rockland County Democrats in which she says, "It's great to be in New York where I come from." Then reporters ask her about the President's reorganization plans, about busing children - a question she neatly unravels by saying she'd rather talk about the quality of education for some children that leads to busing in the first place. The reporters ask about specifics on Carter's hiring of women and minorities. They keep asking her policy questions and justifications for Carter's policies.She explains that her office deals with "organized America" that she's not a policy maker, but the President's "window." And they ask her if she's merely a symbol. "I call it hope, not symbolism," she says. "There are people who are getting into the White House that never got there before."

Later, she's asked about symbolism again. "I think I'm a symbol. Symbols are good because they motivate people." Gone is her joking, her banter. "People have gotten so cynical," she says. "I worked as long and as hard in Rochester as I do in Washington." She talks about freedom and the room seems almost spellbound. She talks about getting over her awe of working in the White House because "what I have is a job and my office is the White House. There are a lot of people who take that seriously. I can tell by the mail."

Back in New York, while waiting for State Sen. Bellamyy, Costanza discovers that there is a "celebrity roast" for Kirk Douglas in the hotel. She wants to go, but doesn't push her way in, deciding that if it's still going on when she comes back that she'll send a note in to Mayor Beam's wife, asking if she can come. Bellamy takes her off to a Junior League of Brooklyn bash where one woman ignores her until she discovers Costanza is a presidential assistant. Then it's off to Queens where Costanza presents an award at the Italian-American organization. By the time she gets back to the hotel, the Douglas party has disappeared.

In Washington, Costanza sees everybody who has a gripe or question for Carter. Because the perimeters of her job are vauge, that means just about everybody who doesn't get to see the President. She or her staff of ten have had meetings with everyone from poet Allen Ginsberg, who wanted to discuss his philsophy on food, to groups who wanted to discuss oppposition to the B-1 bomber.

She puts together memos to the President on the meetings, about what was discussed, which agencies questions or groups have been referred to and the status of these situations. Her meetings from Jan. 31 to Feb. 12 fill a 22-page memorandum to Carter. She sees the President at the senior staff meeting once a week, but the rest of the time communicates by memo.

William Barody Jr., who held the same position as Costanza under President Ford with a staff of 21, says that running the office as Constanza does "technically can't be done.

"You're responsible for everyone in the private sector," he says. "I had someone responsible for each major group, one for Spanish, one for blacks, etc. I tried to involve all the agencies and departments through intensive meetings around the country, usually with 10 to 15 people at a meeting, sometimes 200 people in the East Room when it was a trade group. There were 14 presidential town hall meetings around the country and at 12 of the 14 meetings there are usually a Cabinet Secretary or other top official. Technically, no one person con't do it all."

With all that work, Costanza airily shoots down questions about why there is no man in her life. "I've never met a man who was as interesting as my work." Yet there have been men, probably the most important of whom was John Petrossi, the owner of the cement-construction company where she worked for 24 years, a man who, friends say, Constanza loved and respected and who had a great influence on her. He died last year.

"He was a very dictatorial, very tyrannical man," recalls Carol Clifford, a long-time friend of Constanza's. "He was also different from most of the Italian-American community in Rochester, a member of country clubs and long-time standing in the community. He once told her that she should get the hell out of politics because people were just using her for gofer work. He told her to get into politics for herself or get out. He used to have to throw her out of the office and threaten to fire her to get her to take a vacation."

The other man was a doctor in Rochester. He dated Costanza and frequently they went out with her brother, Peter, and his wife. Each time they would go out, people would call for Costanza, the politico, to come over and speak with them. The doctor decided that while it was nice to go out with Costanza's brother and his wife, he never got to see Midge and probably never would.

What makes Costanza push are memories of "old people when I was a child. I'd see them alone, walking across the street, or waiting for a bus wit ha shopping bag" and she'd wonder whether it was life that had left them alone or whether the government had failed them in some way.

Despite this openness and compassion, Costanza has learned that she can't always be accessbile or available. When she first arrived in Washington, she had a listed telephone number. She got an unlisted one after several 3 a.m. phone calls which she says "were at best, sick."

But with most people, Costanza doesn't have to resort to cutting off communications. One disgruntled White House guard complained about her arranging to meet with a group of gays. So Costanza sent him a note, lightly kidding that a member of the contingent had asked for - and she had given him - the guard's number. Costanza and the guard both thought that was funny - a small but characteristic victory, for the President's liaison with America's special petitioners.