THE NIGHTMARES BEFORE CHRISTMAS

Bill Clinton is usually a great off-the-cuff speaker, able to answer complicated questions smoothly and with a sure command of detail. But at times last week he found himself struggling for words. The worst moment came when a radio reporter questioned the President on vivid new charges about a painful old subject: extramarital affairs. ''So none of this actually happened?'' the reporter asked. The President answered in the tones of a man stumbling through thickets of misgiving. ''I have nothing else to say,'' he declared. ''We . . . we did, if, the, the, I, I, the stories are just as they have been said.'' Finally he arrived at the assertion he might have begun with: ''They're outrageous, and they're not so.'' This was not the way the White House was planning to greet the holidays. After a turbulent but ultimately productive first year, polls were showing that the President's approval rating had jumped to a gratifying 58%. White House aides, looking forward to a long-overdue breather, had lined up a series of Yuletide photo ops and year-end interviews that would let the President and Mrs. Clinton focus on the budget victory, the come-from-behind NAFTA triumph and next year's campaign on health care. The week opened instead with two painful blasts from the past, one about sex, the other about money. The twin controversies prodded back to life old campaign questions about Clinton's judgment, character and trustworthiness. ''We've been having acid flashbacks,'' groaned one official. The most titillating charges, which came to light in the conservative monthly the American Spectator and in the Los Angeles Times, portrayed Clinton as a reckless, obsessive womanizer who used state troopers to arrange trysts even after the presidential election and then tried to bribe potential squealers with offers of federal jobs. The portrayal seemed perilously close to the old ''Slick Willie'' caricature, potentially the kind of story that could seriously damage Clinton's hard-won image as a steadfast, effective leader. Yet the sex stories were probably the lesser of Clinton's headaches last week, because the most credible of them took place before he began to run for President, a period during which he had already admitted that he had caused ''pain in my marriage.'' Far more swampy were new suspicions that the Clintons, as First Couple of Arkansas, had somehow acted improperly while a real estate partner ruined a savings and loan institution that eventually cost taxpayers $47 million to bail out. The Justice Department is investigating the now defunct S&L and the Clinton partnership to see whether money from the thrift was diverted to support faltering real estate schemes, including a development company called Whitewater in which the Clintons had invested, and to finance politicians -- Clinton among them. At week's end the President decided to give Justice all personal documents related to Whitewater, a move that may satisfy investigators for the moment. But the potential conflicts of interest in the case are sure to invite further scrutiny: Hillary Clinton did legal work for the failed thrift, and a Clinton friend served as chief thrift regulator. The shock and gravity of last week's potential scandals had a visible impact on the Clintons. The First Lady reacted defiantly, standing by her man and accusing their accusers of a political conspiracy. ''I find it not an accident,'' she said, ''that every time he is on the verge of fulfilling his