President Carter is nearing his final round of decision making for the coming year's economic policies caught in a difficult box: He is facing an economy that seems tailored for his biggest potential political rival, Senator Edward M. Kennedy (D-Mass.), and there's little Carter can do to counter it.

Just this past week, Kennedy said publicly he will base his decision on whether to challenge the president in the primaries on Carter's handling of the economy. If there is "improvement . . . or at least a perception of improvement by the American people," Kennedy said, he may not enter the race.

The difficulty is, the prospects aren't very encouraging for Carter supporters. As it stands now, the outlook is that the recession will be visible enough to give Kennedy an excuse for challenging the president, but not bad enough for Carter to justify any bold policy shift.

At the same time, if the economy deteriorates further, it can only heighten the Massachusetts senator's advantage. If Carter is forced into a tax cut or other measures, it would give the Kennedy forces more ammunition to use when primary time rolls around. Either way, Carter will be in a bind.

So far, the White House has been cautious about what policies it wants to pursue next January. After raising the possibility of a tax cut last July, Carter now has squelched even private discussion of new stimulus actions -- on grounds that talk of spurring the economy would only aggravate inflation. aggravate inflation.

Indeed, Treasury Secretary G. William Miller stunned official Washington a few days ago by asserting that the nation already is "halfway through" the recession and contending that the economy still hasn't shown any "tremendous strains" -- implying that no new stimulus action will be needed.

At the same time, however, Miller conceded it now seems likely that the jobless rate will rise to at least 7.5 percent by late next year -- from 6 percent last month -- meaning another million-and-a-half persons will be out of work before the other "half" of the recession is over.

If Miller's forecast proves correct -- and officials continue to dismiss the recession as ended -- it could give Kennedy just what he needs for an issue. (The Massachusetts senator conceded Thursday he has no major qualms over Carter's economic policies. It's just his style of leadership, Kennedy said.)

Moreover, Miller's assessment still is more bullish than those of most other forecasters, either in or outside the administration. Most now see a visible, albeit still relatively shallow slump ahead with the jobless rate rising to between 7 and 8 percent by next fall.

By even the most optimistic of these consensus forecasts, the jobless rate still will be rising rapidly at the end of this year, when Kennedy is slated to make his decision formally. There may be a sudden slowing in inflation, late this year, but analysts say much of it will be temporary.

Moreover, the adminsitration faces the prospect of indefinite tight-money policies by the independent Federal Reserve Board, which itself is hemmed in by the continuing threat to the dollar. Each further hike in interest rates increases the danger that the tight-money policies will bring on a crunch.

The Fed's tightening already has begun to bite, and there are fears that the bank -- which has become visibly more tough-minded under new chairman Paul A. Volcker -- may hold tight too long, the mistake it has made most often in past recessions. If a pinch comes, it could be just before the election.

Even with the recession, there's little the administration can do to really slow inflation in 1980. Economists agree that even a moderate slump won't do much to trim the current 13 percent rate below the 8 to 9 percent range -- and that would be due largely to an easing in energy price hikes.

Most of Carter's highly touted "austerity" policies -- if he can maintain them in the face of mounting expenses related to rising unemployment figures -- are little more than holding actions.

With that in mind, there are few who fault the president for refusing to rush into a tax cut. Until only recently, there have been few signs that the recession had advanced beyond the forecast stage. Although output had declined, the jobless rate hadn't begun to rise. And most other indicators were mixed.

Officials argue with some credibility that to unveil a tax-cut package now, virtually would invite Congress to shuck all fiscal discipline, sending a signal to the markets that the government had abandoned the anti-inflation fight. Moreover, it isn't clear how big a tax reduction would be needed.

In the past few days, however, there have been growing indications that the forecasters may prove right, both about the recession and the inflation rate: The jobless rate in August jumped three-tenths of a percentage-point, to 6 percent -- its highest level since January -- the Labor Department reported.

And wholesale prices, often a precursor of long-term inflation trends, soared 1.2 percent in August -- their fastest pace since February. At the same time, other indicators published last week showed inventories continuing to swell, raising the possibility that the recession may prove deeper.

As in previous years, a substantial part of the current problem stems from the president's own actions in managing the economy. By hiw own advisers' admission, Carter overstimulated the economy in 1977 and 1978 and did not pay enough attention to inflation. Those indiscretions are coming home to roost.

But a good deal of the difficulty this time also stems from factors beyond the president's control -- the sharp run-up in crude-oil prices, for example, and at least part of the decline in the dollar. As a result, the slump probably would have come no matter what.

The danger for Carter may be in the timing of the downward slide. If the economy deteriorates rapidly, as some forecasters are predicting, it may deprive the administration of a chance to put its proposals together in time to bite without exacerbating inflation.

The traditional problem with anti-recession programs has been that the stimulus too often has come after the economy has begun its recovery -- too late to prevent unemployment from rising, but just in time to create new inflation pressures that all but blunt any "benefits" the slowdown may have brought.

How to balance all these considerations -- and still beat Kennedy in the coming primaries -- will be difficult enough for the president. But the box could be worsened by a range of outside events: Another sharp increase in crude oil prices -- or a further decline in the dollar -- could send inflation up again.

Meanwhile, the consensus among forecasters is that the downturn -- and the current high inflation -- will continue for the next several months.

Most analysts are predicting:

The economy will continue to slide, possibly through the first quarter of 1980, with the severity depending on a spate of uncertain factors from the extent of the current buildup in inventories to the prospect of an auto strike. The recovery, in spring and early summer, will be decidedly weak.

Inflation will continue to rage at an uncomfortable pace, dipping below the current 13 percent annual rate, but unlikely to slow beyond a still-virulent 8 percent or so. (By comparison, consumer prices in 1978 rose by 9 percent.)

The jobless rate will rise steadily, to between 7 and 8 percent by the time of the November election -- raising the prospect that between 1.4 million and 2.4 million more persons may be out of work by the time the voters go to the polls.

It's still too early to tell just what, if anything, Carter will propose in his January policy package.If a tax cut does emerge, the betting is that it will involve, as Carter has hinted, a rollback of scheduled Social Security tax increases and a spur for business investment. But the scope is still in doubt.

On the spending side, Carter most likely will have to abandon again his pledge for a balanced budget. The prospect of automatic increases in unemployment benefits, combined with Senate demands for heightened defense spending, will make that impossible to fulfill.

The difficulty is, no one quite knows for sure just where Carter will come out in the current round of Policymaking. After 2 1/2 years of shifting and backfilling, the administration still has no visible underlying economic philosophy, and what it has changes from quarter to quarter.

To many analysts, the blithe dismissal of the recession's prospects expressed by Miller the other day may make for tough-sounding rhetoric to prevent Congress from pushing through a tax cut this fall, but it doesn't address adequately the basic economic realities.

In almost the same breath in which he asserted the recession was half over, the Treasury chief conceded that the jobless rate was still likely to rise to at least 7.5 percent by the end of next year -- meaning another million and a half persons will be unemployed before the other "half" is over.

The plain fact is, Carter once again is caught in a box, and this time the stakes are more serious. Not only is the president's re-election hanging in the balance during this autumn's decision-making process; what Carter decides now could affect the health of the economy for years.

Meanwhile, the government's economic policy is being set by Volcker's Fed, which has been boosting interest rates steadily despite the risk that it may be worsening the coming recession. The prime interest rate soared to a record 13 percent last week, and is expected to climb even higher.

So far, top administration officials have stood by silently and watched, providing the rationale that with the dollar falling a few weeks ago, Volcker all but had to raise interest rates at first, and that he may begin now to ease off.

Miller told the National Governors' Association here last week that "I think Paul has pursued an inevitable policy" at the Fed, "good, bad, I don't know, but . . . inevitable." A few days later, he asserted he didn't believe interest rates were likely to "be out of kilter" and worsen the recession.

But less sanguine officials concede that Carter soon will have to begin making decisions that will determine not only the economy's fate but his political future as well. The danger for him is that the current no-win economic situation could become a political defeat as well.