AS RONALD REAGAN enters unsteadily into the third year of his presidency, it has become increasingly fashionable to pronounce his administration a failure that is doomed to be rejected by the country in 1984. But despite the apparent failure of Reaganomics, despite the worsening recession and mounting unemployment, despite the huge federal budget deficits in prospect, such political forecasts may be premature. It is entirely possible that the Reagan White House will "stay the course," and in the process win both a second term and most of the Reagan agenda. This result may have unsettling consequences for American society, but who said this is the best of all possible worlds?

How could a failed economic policy be rewarded with a majority vote? Consider a plausible scenario for the Reagan reelection pitch in 1984:

* Inflation remains at 6 percent or less. The president reminds voters that whereas prices were skyrocketing under Carter, he has cut the rate of inflation in half.

* Unemployment has nudged down to 9 percent, still leaving more than 10 million unemployed. Reagan claims that less than 3 million Americans are really unemployed claiming that a 6.5 percent jobless rate is "natural" in a modern economy.

* Interest rates have continued to show some modest declines, enabling more affluent consumers to purchase homes, cars and other big ticket items with 12 percent loans. The president evokes memories of the recent past and declares victory.

* Despite a defense buildup more modest than he originally proposed, Reagan declares that the "window of vulnerability" has been closed. Stressing Democratic opposition, he touts himself as responsible for making America "numero uno" on the battlefield.

* A program to rebuild the nation's infrastructure, financed by regressive "user fees" instead of less palatable tax hikes, demonstrates the administration's concern for national needs. Members of the hard-hit building trades and other unions are persuaded to support a second term.

* Due to the continuing recession, some state or city -- take your pick -- goes bankrupt. President Reagan uses this misfortune to remind voters that we are still paying for the sins of the "big spenders."

* Deficits persist, but the panic of further deterioration subsides as compromises are struck on modest spending cuts and revenue increases. Arguing that the most difficult adjustments are over, corporate leaders close ranks solidly behind the administration.

The list could go on. President Reagan could take steps to convince the New Right that he is still the best person to fight for social issues such as busing, prayer in schools and anti-abortion legislation, while profiting from squabbling and disunity within Democratic ranks. To be sure, he would need some breaks, but a scenario like this one is hardly out of the question.

It's not impossible to imagine a pro-Reagan coalition in 1984 consisting of traditional Republicans, the New Right, many blue-collar workers and even some minority voters. With such a coalition and all the financial support he can use, the president could win four more years.

It is important to keep in mind that the president does not need a sound economy on which to construct his reelection platform -- only one that's perceived as on the verge of renovation.

Never mind that the administration has no anti-inflation program consistent with vigorous economic growth; an oil glut and excess capacity may be adequate to restrain prices for another two years.

Disregard the 10 million workers who would still be without jobs, because voters have proven tolerant of high unemployment and are more concerned about its apparent direction than its absolute level.

Remember that Reagan can always avoid full responsibility for high interest rates, as the Federal Reserve is an independent agency. President Reagan has shown a remarkable propensity for finding culprits.

Where the administration's record is found wanting in 1984, Reagan will be able to fall back upon a clear world view with identifiable villains. Blame can be apportioned to intrusive government, bleeding-heart liberals, the Soviets and Congress. In contrast, the Democrats seem unlikely to offer an explanation for what ails us (not to mention whose fault it is) which can be grasped by a majority of voters. So once again, the anti-big government drums could be beaten, and for a second time the Reagan ideology could play well before numerous audiences.

The idea that a failed economic policy could be affirmed by majority vote is anything but comforting. Yet even more disturbing is the realization that a successful Reagan coalition in 1984 would be built from the top down. By their very nature, the president's campaign appeals would have a divisive undercurrent, emphasizing self-interest with "them-or-us" themes. And for the losers in society's bottom half, there would be little hope, for victory would be a mandate to ignore their claims.

Besides the poor, another group of losers, albeit less needy, would also be wounded if this scenario for a second Reagan victory comes true. They would be those Americans who prefer the ideal of a unified society to a contest between "haves" and "have-nots." They would be those who believe coalitions should be built from the middle, and sufficiently broad to consider the needs of those below as well as those above.