WHEN Judge Clarence Thomas and law professor Anita Hill appear today before the Senate Judiciary Committee, at issue will be an offense that is widely alleged but little understood, particularly by men.Since the debate on sexual harassment burst onto the national scene this week, Professor Hill's charge that Judge Thomas sexually harassed her 10 years ago has elicited outrage and sympathy among women. Those questioning the timing and validity of her charges have tended to be men. It was an all-male judiciary committee that was prepared to let the FBI report detailing her remarks slide by and for Thomas to be confirmed Tuesday evening, until an aide to a committee member reportedly leaked the information to the press and the issue exploded. "A Pandora's Box of outrage has been opened," says Eleanor Smeal, president of the Fund for the Feminist Majority. "We have the power; we just have to use it." Hill has charged that Thomas created a "hostile environment" at their work place by repeatedly describing pornographic scenes to her after she refused to go out with him. Such verbal harassment violates guidelines of the Equal Employment Opportunity Commission (EEOC), which Thomas headed at the time. Hill is not alone in believing she has been sexually harassed on the job. Numerous surveys have shown that a sizable percentage of working women say they have suffered sexual harassment, though very few have filed lawsuits. In a 1988 survey by the United States Merit System Protection Board, 42 percent of women employed by the federal government said they had been harassed in the previous two years. Of those, only 5 percent filed complaints. A survey conducted for the House Armed Services Committee found that 80 percent of women employed by the Department of Defense reported harassment. The results were rejected as suspect, and another survey was taken. The questioners asked first about teasing, then harassment - and got the same result. Another 1988 survey, of Fortune 500 executives queried by Working Woman magazine, found that one-third had experienced sex-discrimination lawsuits. One-fourth had faced repeated suits. EEOC figures for sexual-harassment charges filed between 1985 and 1990 show that levels have not gone up or down dramatically during that period; the number of charges ranged between 4,446 and 5,572 a year. The number of EEOC lawsuits filed over sexual harassment increased slowly between 1986 and 1990, from 38 cases to 50 cases. But even getting courts to recognize sexual harassment as a crime is a fairly recent gain. When it was first recognized, it was only in the most severe cases - for example, if a woman failed to receive a job because she would not perform sexual favors, says Diana Johnston, an EEOC lawyer. By November 1980, EEOC guidelines on sexual discrimination spelled out the "hostile environment" construct. Those guidelines cited a district court case from 1978 that recognized creation of a hostile environment as a form of sexual harassment, which is one of the earliest such decisions, says Ms. Johnston. One of the problems for women faced with sexual harassment is that usually there are no witnesses, which may explain why so few women press charges. Of the cases taken to court, however, few come down to the man's word versus the woman's word, probably because many women are not willing to jeopardize their careers and their dignity by pressing an embarrassing case they are not likely to win. "If it does boil down to 'he said, she said,' from what I can see in court the woman tends to lose out," says Johnston. The Senate's lack of awareness of the kind of bombshell it was sitting on may be explained in part by the culture of Capitol Hill, often described by aides as "the last plantation." Though Congress creates the laws on hiring and firing practices, it is not governed by them. The House, instead, falls under the jurisdiction of the House Office of Fair Employment Practices, which has a grievance procedure. The Senate has no rules, though some offices have adopted their own standards for acceptable conduct. The congressional women's caucus has lobbied all Senate offices to do this. Congresswomen have revealed that even they encounter inconsiderate male behavior. "It was not too long ago that a colleague of mine complemented me on my appearance and then said he was going to chase me around the House floor," Rep. Jill Long (D) of Indiana told the House Wednesday. "Because he was not my boss, I was not intimidated. But I was offended and embarrassed. Sexual harassment is serious; it's not funny and it's not cute and it certainly is not complimentary."