THE undoing of Zoe Baird's nomination for US Attorney-General, the top law-enforcement official, is richly revealing. It exposes an undercurrent of class resentment against shakers and movers who exempt themselves from the rules they expect others to live by. It highlights, once again, the urgent need for day-care in an era of working couples and single parents. And it serves notice to Bill Clinton that his presidency is tethered to public opinion by a short leash, hope and enthusiasm notwithstanding. Ms Baird's defeat last weekend also suggests a double-edged double-standard when it comes to women in politics. Surely it's true that a man with her narrow qualifications would never have been selected to be Attorney-General. But surely it is likewise true that a male nominee would not have been politically decapitated for so commonplace a crime: employing two undocumented workers as domestic servants. Most of all, perhaps, ''Nannygate'' reminds us of a deep and growing ambivalence in America's attitude towards immigrants, illegal or otherwise. Among the most durable of American truisms is this: the US is a proud patchwork quilt of immigrants who come to its shores in search of fortune, in flight from persecution, or both. The Statue of Liberty stands as a sentimental symbol of equal opportunity, a siren beckoning the world's tired and hungry, its poor and huddled masses, an affirmation of the nobility of humble origins. The US Government has frequently declared blanket amnesties for long-time illegal residents, asking only for documentary proof that they have lived and worked here for a certain time. More recently, the state has even initiated an annual visa lottery, giving away 40,000 green cards to the lucky 1-in-100 whose names are drawn. (The pool includes 16 countries with 13,000 slots reserved for citizens of the Irish Republic and Northern Ireland. The only Asian nations on the list are Indonesia and Japan.) We like to think all Americans were immigrants once, and thus should embrace all who follow. Discrimination becomes, in this light, an act of hypocrisy. Indeed, that we are a nation of immigrants has become part of the national mythology. But this particular myth fulfils both meanings of the world: it is not only a sense of self-identity, but a half-truth, at best. Native Americans, to begin with, have no place in this rose-tinted vision. Nor do many black Americans, whose ancestors experienced America as a land of oppression not opportunity, have reason to embrace the immigrant myth. But even beyond these aggrieved groups, there is abundant evidence that Americans are in two minds about those knocking on the doors of opportunity, and especially those who have already slipped in through the cracks. There are millions of people - no one knows just how many - working and living in the United States without proper documentation. The government classifies them as ''illegal aliens''. What it gives with one hand - amnesties, the visa-lottery, political asylum - it can, and does, take away with the other. More than a million ''illegal aliens'' are apprehended and arrested every year, mostly from Mexico, the Caribbean and Central America. Most are sent home. More recently, many Chinese and Vietnamese immigrants - smuggled into the country by organised crime syndicates based in the US, Hongkong and mainland China - have also been detained. True to form, the government does not always apply its heavy hand evenly, even according to its own criteria. In theory, political refugees - those who fear persecution or death at the hands of their own government if they return - get to stay, and economic refugees - those who come just for the money - are sent back to where they came from. In practice, a more reliable determinant of whether someone gets deported has been whether he or she came from a communist country. Few citizens of the People's Republic of China, for example, who set foot on US soil have been repatriated. (They may be wasting away in detention centres, but they haven't been sent back to China.) It is not clear how the end of the Cold War will affect this unacknowledged criterion. Nothing better encapsulates the strange status of undocumented workers in the US than an article in a New York newspaper last year. ''Green Card Applicant Aid: Meeting for Illegal Aliens,'' read the headline. ''These public information meetings are scheduled this week for illegal aliens who want to apply for a green card in the lottery later this year.'' Where else in the world do refugees from the law convene in public to discuss their illegal status? Popular attitudes are ambivalent, too. While it is obvious that many illegal immigrants take jobs that most Americans shun (farm work, child care, house cleaning, dish-washing, etc), rising unemployment inevitably breeds resentment. Ms Baird's case brought howls of protest on radio-talk shows and in letters to Congressmen about foreigners stealing American jobs. Instance of violence against Asian immigrants in particular - resented by some for their relative success in moving up the economic ladder - have increased over the last decade. Some of these attacks are also motivated by racism, although they remain far less frequent than, for example, in Germany. But as the number of illegal immigrants continues to rise, so too will tension in society and pressure to revise the 1986 Immigration Reform and Control, Act, which has failed, by most accounts, to stem the flow.