(FORTUNE Magazine) – American consumers are back -- at least they think they are, to judge by their recent behavior at cash registers. In the holiday period leading up to President Clinton's inauguration, confidence and spending both boomed. But the euphoria could dissipate as suddenly as it arose -- as soon as Americans realize how little they actually have in their wallets. The two most widely followed measures of consumer confidence both shot up after last November's election. By January, the University of Michigan consumer sentiment index had made up all the ground it lost during the prior two years. Meanwhile, the Conference Board index registered what the Board called ''an imposing gain'' pointing to ''significant improvement'' in the economy. Consumers put their money where their polls were. After several years of Scrooge-like behavior, shoppers spent a bundle at Christmas. Retailers didn't have to slash prices to entice them either. People were only too glad to dust off their credit cards and dig into savings. VISA U.S.A., for example, says charges hit an all-time peak for the Christmas season. Personal savings fell to a 20-month low of around 4% of disposable income, according to government data. Consumers weren't buying only gifts. Matt Howard, senior vice president of marketing at Sears, says every line of big-ticket durables, from furniture to home fax machines, did well. He observes: ''Customers no longer hesitate to make long-term commitments.'' Similarly, home products such as furniture and rugs led December sales gains at Federated Department Stores; demand for costly consumer electronics items, such as large-screen TVs, increased sharply. At J.C. Penney, December sales rose a booming 11.6%, with men's clothing the biggest gainer. The party extended from socks to -- you guessed it -- stocks. Americans poured a record $9.9 billion into equity mutual funds in November, the most recent month for which there are data. Investors' intentions to buy equity funds hit a three-year high, according to an ongoing University of Michigan survey paid for by Fidelity Investments, the big mutual fund company. Meanwhile, November sales of existing homes soared to the highest level since 1986. The surge in optimism comes, surprisingly, when real hourly wages are still declining and the job market remains weak (see charts). One explanation is post-election excitement: Richard Curtin, director of the University of Michigan survey, says Americans were turned on by the prospect of change. Yet this is only the latest in a series of consumer mood swings that puzzle the experts. Says David Wyss, research director at DRI/McGraw-Hill, an economics consulting firm: ''Confidence went from abnormally depressed to overly happy without stopping in the middle.'' Wyss worries that if the mood shifts again, people will quit spending and stunt the economic expansion. What might undermine consumer confidence? For starters, the President could make decisions that consumers won't like, such as reneging on his pledge to cut taxes on the middle class, as now seems likely. Says David Munro of High Frequency Economics, a consulting service that advises bond and currency traders by fax: ''Clintonomics will have irritants for everyone.'' Wage earners will also be surrounded by reminders that times are still hard. Employment growth continues to creep as businesses keep trying to squeeze more productivity out of their workers. U.S. companies now employ 1.6 million fewer people than they did in mid-1990, and many positions have been permanently eliminated. It now takes an average of 19.2 weeks to find a new job, vs. 15.5 weeks a year ago, according to the Bureau of Labor Statistics. Nor will Clinton have much immediate impact on other stubborn fundamentals that put a drag on prosperity: the huge surplus of commercial real estate, the defense build-down, and the massive federal deficit. SMALL WONDER that businesses are not as ebullient as their customers. The American Business Conference, which consists of fast-growing medium-size companies, found in a recent survey that its members expect a ''fragile'' first quarter. Only 34% of respondents plan to increase employment in the period, while 25% intend to reduce payroll. And those are growth companies. Small businesses, which everybody thinks will remain the nation's main job generator, don't intend to hire much in the near term either, according to a December survey by the National Federation of Independent Business. Even retailers who enjoyed their most festive season in years aren't exactly donning gay apparel. Federated vice president of corporate affairs Carol Sanger says that despite the surge in consumer confidence, the company expects a first-quarter slowdown ''because some of the energy will have been sucked out by the strength of Christmas.'' Kmart is cautious too, says a spokesman, out of concern that consumers will opt for paying off debt rather than continuing to spend. Like many retailers, Jack Goodall, CEO of Foodmaker Inc., usually depends on consumer confidence as an indicator of future sales. But even though his Chi-Chi's restaurant chain saw a substantial increase in year- end business, he isn't ready to believe that revenues, down by 2% in fiscal 1992, have turned around. $ Business is right to remain cautious. Even if consumer confidence keeps rising, FORTUNE believes, people won't be able to accelerate spending. Both income and consumer spending will grow a modest 2.5%, vs. 2.1% in 1992. The job and wage growth just aren't there to support sustained shop-till-you-drop behavior.

BOX: OVERVIEW

-- Shoppers rev up spending to match their upbeat mood.

-- But they're buying on credit, and saving less.

-- Job and wage growth is too slow to sustain the spree.

CHART: NOT AVAILABLE CREDIT: FORTUNE CHART CAPTION: CONFIDENCE HAS SURGED. . . BUT WAGES KEEP DECLINING. . . AND EMPLOYMENT IS LOW