US to First Ladies: wear a hundred hats, just don't buy them. The First Lady's job mirrors the expanding role of women. But White House spouses walk a tight rope between political concern and showing too much political prowess.

Dolley Madison promoted the rebuilding of Washington after it was burned by the British. Edith Wilson traveled to Paris after World War I to observe the signing of the peace treaty (which she did seated behind draperies).

Eleanor Roosevelt held an official government position.

Rosalynn Carter attended Cabinet meetings.

Nancy Reagan acted as host of an international conference on drug abuse.

From Martha Washington to Nancy Reagan, the nation's ``First Ladies'' have played an important role in the functioning of the US presidency. And, according to James Rosebush, Mrs. Reagan's former chief of staff, that role is expanding.

``Today, with the evolving role of women in the workplace, the rising expectations placed on women in society, the increasing degree to which women are involved in substantive decisionmaking from household management to foreign policy, the duties of the First Lady have changed dramatically,'' says Mr. Rosebush in a slim new volume called ``First Lady, Public Wife.'' ``No more is she just an official hostess for the White House. Recent presidential marriages have reflected contemporary views of what is an acceptable sharing of responsibilities between married partners.''

The 1988 presidential race is focused almost exclusively on the men aspiring to sit in the Oval Office. But judging from history, the wife of the aspiring candidate also deserves scrutiny. For she can be either an asset or a liability in performing the demanding ``job'' of First Lady.

Rosebush defines the functions of that job as ``manager, diplomat, hostess, champion of causes, political partner, wife, and mother.'' But each First Lady writes her own job description, and the task is not an easy one, given the ambivalent attitudes of the American public.

``People want to idealize the First Lady,'' Rosebush says, ``but they don't want her to idealize herself. They want her to work on causes but to save enough energy to entertain impeccably, speak foreign languages, travel comfortably (but inexpensively), and maintain a happy home life. And what's more, they are ready to criticize severely any First Lady who cannot perform well in most of these functions or any First Lady who oversteps in the interpretation of her role.''

Not all First Ladies - and there have been 51 - espoused social causes or became political confidantes of their husbands. Many concentrated only on their families and on presiding over social functions, a huge undertaking in itself. A few, like Martha Washington and Louisa Adams, found the presidential life either dull or constricting.

But many of the best-remembered First Ladies relished their role and sometimes widened their activities in impressive ways. Ellen Wilson was the first to take on a social project, promoting congressional legislation to improve the ramshackle back alleys of Washington that housed the city's poor blacks. Grace Coolidge helped wounded veterans in World War I. Eleanor Roosevelt took on the cause of racial justice at a national level and held press conferences.

Mrs. Reagan's antidrug crusade began in 1979, when campaign advisers urged her to become identified with an issue. Coming out of a medical background (her father was a doctor), she herself chose the field of drug abuse, and, although politics was the initial motivation, ``the problem drew her in'' and ``changed her,'' writes Rosebush. She was the first to hire a staff of experts and in the past eight years she has logged tens of thousands of miles and untold hours giving speeches. She also refused to be a spokesman for the government's antidrug effort, electing to focus entirely on private-sector efforts.

``No one cued her to take this approach, it was instinctive,'' says Rosebush, who directed Mrs. Reagan's staff from 1982 to 1986.

Mrs. Reagan has sparked considerable comment about her political influence at the White House. But as her former chief of staff notes, her ambitions were for the President, not herself. During the 1976 and 1980 campaigns Mrs. Reagan often attended strategy meetings. After the first presidential debate, she suggested a new approach to the President's advisers. She has also brought pressure to bear on personnel decisions, most recently during the resignation of former White House Chief of Staff Donald Regan.

``Her interest lay in having effective people serve her husband,'' Rosebush says.

She was almost ``crucified'' by the news media as power hungry after the Regan episode. But, says her former aide, her ambition resulted from a ``built-in defense mechanism that surfaced when her husband was threatened.''

Other First Ladies have similarly played the role of ``political partner.'' Abigail Adams was so vocal in her opinions she was dubbed ``Mrs. President'' and ``Her Majesty.'' Sarah Polk and Mary Todd Lincoln also engaged in direct lobbying. And Eleanor Roosevelt became a representative of the President with her far-flung activities. As for Rosalynn Carter, she was a ``thoroughly political First Lady who was treated as an equal political partner by the President.''

But there are hazards in stepping out front politically. Rosebush observes that the American people do not want a surrogate president whom they did not elect. Any First Lady has to be careful about exercising political power, the author says, and to date no First Lady has abused her influence.

As the First Lady's responsibilities have grown, so has her staff. Edith Roosevelt was the first to acquire a permanent nondomestic staff person to help with personal mail. The job of press secretary came into being with Jacqueline Kennedy, when television began making greater demands, and a ``projects manager'' was added when Lady Bird Johnson's beautifica-tion project got under way.

Over the past few administrations the First Lady's nondomestic staff has leveled off at about 20 people.

So demanding and complex has the job of First Lady become that Rosebush suggests Congress may one day provide her with some financial compensation, just as ambassadors' wives now receive limited pay. ``It may be time to acknowledge the fact that most official jobs require hard-working spouses,'' he writes.

If the wives of today's presidential hopefuls wonder just how hard, they might ponder this capsule description from ``First Lady, Public Wife'':

``Any First Lady must be, according to public dictates, attentive and interested in anyone - farmers, factory workers, and kings - but not too familiar. She must be well informed about her husband's political agendas, regardless of her interest in them. She is expected to reflect the spirit of America - the whole country - without regard to her own regional background. She must treat the press without contempt, even though they can treat her with it. Be well dressed, not overdressed. Be prepared, not rehearsed. Be a devoted wife, but also a woman of independence. Express opinions frankly, but never be flippant. Care for the house, but never appear to be absorbed in it. Show emotion, but never appear contrived.''