It has not been exactly a banner year for Republican women. The only Cabinet-level job President Reagan handed to a woman went to a Democrat, Jeane Kirkpatrick, the ambassador to the United Nations. Sub-Cabinet and senior White House staff jobs for women have been as scarce as -- you should excuse the phrase -- hens' teeth.

But last week a Republican woman achieved something unprecedented. Nancy Sinnott, 30, was named executive director of the Republican Congressional Campaign Committee. So far as memory and records show, she is the first woman to fill the top professional staff job in any of the "Big Three" political committees of either party. Sinnott will be running a staff of 40 people and managing a budget of more than $35 million in the two-year effort Republicans will make to end the Democratic majority in the House of Representatives in 1982.

Each party has had a woman national chairman once in its past. Jean Westwood was put in the Democratic chairmanship by George McGovern and Mary Louise Smith was named Republican national chairman by Jerry Ford. While protecting their patron's interests, they both hired their own pros to manage their shops. Neither the national committee, nor the senatorial campaign committee, nor the congressional committee of either party had previously found a woman it thought fit to fill the top job reserved for a full-time political pro.

In that sense, Sinnott's appointment is another landmark in the slow but certain emergence of women as full partners in the working of the American democracy.

What the committee chairman, Rep. Guy Vander Jagt (R-Mich.), who recommended Sinnott to succeed Steve Stockmeyer, says about her reflects the standards by which all women in politics would like to be judged.

"Nancy earned the right to the job," Vander Jadt said. "She's done her other jobs here so well, she was the only choice." Sinnott has been on the committee payroll for four years, the first two as a field representative in New England and Illinois, the last two as campaign director -- the deputy to the retired Stockmeyer.

"Being campaign director is the toughest job in the whole committee," Vander Jagt said. "Every Republican representative and senator is putting pressure on you to back their own favorites, even when the objective evidence indicates the money and resources should go to other races. It takes tremendous strength to say no to all those people, and she is as tough as steel.

"But even when they'd been turned down by her," Vander Jagt added, "they'd say, 'She's quite a lady.'"

Sinnott, whom an old chauvinist like myself would describe, if I dared, as a slim brunette with the looks of the young Joan Bennett, is another kind of rarity in today's Republican capital: an avowed feminist. The child of a Chicago businessman, she went to Wheaton College, in Massachusetts, also taking courses at Williams College.

It was in the liberal tradition of the underdog Massachusetts GOP that she received her political training. While still in college, she worked on the campaign staffs of Gov. Francis Sargent and Lt. Gov. Donald Dwight and then in their offices, assisting in reorganization of the state's sprawling executive agencies into a Cabinet-style government and in the implementation of the school racial-balance decisions.

At the urging of then-senator Edward W. Brooke (R-Mass.), she ran for vice chairman of the state GOP in 1975 and defeated three opponents for the job. In 1976, she managed Arthur Mason's House campaign against then-representative Robert F. Drinan (D-Mass.). Although Mason lost, his campaign drew more Republican votes than any other race any Republican has ever run in that district.

It was the Mason campaign that brought her to the attention of the Republican Congressional Campaign Committee, a powerhouse political operation that every two years sets new records for its fund-raising and campaign services.

As campaign director and now as executive director, Sinnott has helped recruit and elect Republicans of all varieties, working as hard on the campaign of Clay Shaw, a Florida conservative, as on that of Claudine Schneider, a Rhode Island feminist -- both of whom were elected last fall.

When told that one right-wing political action committee director regards her as a libertarian while another describes her as a liberal, Sinnott does what any smart pol would do. She laughs and says, "Well, actually, I'm neither. I'm a Midwesterner like Dave Stockman, and it's easy for people like us to be progressive on foreign policy and civil rights, pragmatic in politics and conservative in the economic area. Fiscal sanity and personal freedom look to me like they go hand in hand, and I don't want the government so out of check that it threatens either one of them."

In a season when they have few victories to cheer, a great many women, including Democrats who have no desire to see Sinnott succeed too well in her job, are offering congratulations to her for getting there.