This month all of Washington and most of America seem to be wrapped in a bubble of euphoria inflated by Ronald Reagan and David A. Stockman. Even more than most bubbles, though, this one is fragile. It could be burst by any one of many sharp obstacles already visible on the economic horizon.

Poor economic conditions made Reagan's election possible, and Reagan and Stockman have now launched a budget- and tax-cutting crusade to improve those conditions. Optimism that the crusade will succeed in Congress and will turn the economy around is the basis for the new euphoria.

But Richard Wirthlin, Reagan's pollster, has warned Senate Republican leaders that the public expects tangible results from Reagan's economic plans within six months. The new Washington Post-ABC News poll this week also showed most Americans expect to be substantially better off a year from now. These expectations are part of the current euphoria, yet even some economists inside the new White House staff doubt they can be fulfilled.

Officials whose job it is to predict the course of the national economy privately predict at least two quarters of "weak" economic activity this year, a performance weaker by far than any public projections from the White House suggest. A faltering economy will undo the administration's public economic forecasts, significantly increase the budget deficit and conceivably undermine confidence in the entire Reagan program. One administration economist called the unnerving probability of a "weak" economy this year "our Achilles' heel."

Many Democrats in Congress privately predict that in six to 12 months the president will be on the defensive, trying to explain why inflation and interest rates are still high and why his tax cuts haven't really put any new spending power into the hands of middle-class citizens.

The factors that even some sympathetic experts say could undo the administration's forecasts -- and conceivably undermine confidence in the Reagan program -- include these:

Energy costs. In his now-famous "Dunkirk" memo, Stockman predicted a sharp tightening of the world oil market later this year. Tighter markets mean higher prices, and -- as the last seven years have made so clear -- oil price increases reverberate through the American economy like dodge 'em cars in the amusement park, exacerbating inflation.

Food costs. The international grain market is tight for 1981, and could get much tighter, pushing prices upward in America. Food prices had a substantial influence on the inflation of the '70s, and could reassume that role at any time.

Interest rates. The Federal Reserve System has just announced plans to restrict even further the supply of money. In recent years, such moves by the Fed have tended to push interest rates upward as borrowers competed for declining quantities of available capital. The administration projects instead that the tax reductions it is proposing will induce billions in new savings, adding to the supply of capital, but this is a controversial guess that is disputed by many economists. The administration's economic forecasts are dependent on sharply falling interest rates; if they don't materialize, neither will the good economic news being predicted at the White House.

Depressed sectors of the economy. The auto business, so central to the American industrial market, is stuck in a depression that could easily continue. It is the product of high sticker prices, high interest rates, customer dissatisfaction with the products being offered and general consumer uncertainty. Housing starts are also off. Any new boom would require a revival of both these sectors.

International economic conditions. There are already disquieting signs here. The dollar's value has risen quickly in recent weeks, a trend that works against U.S. exports and encourages imports, thus doubly dampening economic activity in this country. West Germany's fabled economy is stagnant, a sign of a European recession that also has negative implications for the increasingly interdependent U.S. economy.

Apart from these concrete difficulties, the Reagan administration's plans are based on admittedly optimistic theoretical models that could easily fail to pan out. One is that businessmen will make literally unprecedented investments in plants and equipment at a time when government policy is substantially reducing the amount of money in circulation, and thus -- potentially -- the net demand for new goods and services.

In effect, to fulfill the administration's hopes, American businessmen will have to decide that underlying economic trends are so favorable that they should expect increased demand for good's and services soon, even if demand is obviously going to be declining in their near term.

Reagan economic aides also theorize that inflation is caused largely by inflationary expectations in the marketplace; they want to reverse those expectations. But it took Americans a decade to learn to take inflation for granted and to count on its continuation; how quickly can those habits of mind be changed?

Inside today's bubble of euphoria, politicians and the mass media are diverted by the spectacle of the budget-cutting crusade. Stockman and Reagan are playing lions, the special interests are the Christians, and it's sure to be a bloody good show. The polls and the politicians agree that for the moment, public opinion overwhelmingly supports the new president.

But the Post-ABC poll showed that in order to join in the general euphoria, a great many Americans have decided to disagree with themselves. The poll asked respondents if they thought a range of government programs, from Medicare to synfuels subsidies, should be cut, and the answer was overwhelmingly no. Later in the poll the same people were asked if they supported Reagan's budget cutting, and the answer was a resounding yes (by more than 3 to 1).

How can Americans be so contrary minded? Partly by believing -- as a new Gallup Poll reports -- that 48 cents of every tax dollars is wasted by the federal government. If waste is so rampant, it should be easy to cut the budget without cutting those popular programs, right?

Public opinion may not remain euphoric when voters realize that the cuts will hurt them, and when the national economy refuses to improve as fast as people now want and expect it to.

If his bubble bursts, Ronald Reagan will face a political problem akin to Franklin D. Roosevelt's during the 1930s. Roosevelt never could end the depression until World War II broke out, but he kept on winning elections anyway, FDR's political success suggests that Reagan does not necessarily have to meet all of his economic goals to maintain his political popularity, provided the voters continue to believe in the efforts he's making.

But Roosevelt took the generous route, giving away money and jobs as he went. In the short term, at least, Reagan proposes to take away jobs and money, which may not be as popular.