Question now is not 'will recovery cool?' but 'by how much?'

Economic forecasters are no longer arguing over when the economy will slow down. The discussion now focuses on how sluggish the growth will be in 1985 and what the effects of the slowdown will be.

Last week's drop in the prime, or benchmark, lending rate to 12 percent is just the latest sign that the pace of US economic growth is cooling, economists say.

The consensus seems to be that the economy will not slow enough to trigger a recession, a period when economic output falls.

''The recent declines in interest rates will get 1985 off on a better footing than could be expected even a month ago,'' says Robert Gough, senior vice-president of Data Resources Inc. (DRI).

But economic gains will be tepid enough that the nation's unemployment rate will either remain at current levels or rise a bit. Economists generally figure the economy must grow at a 3.0 to 3.5 percent annual rate (after adjusting for inflation) to absorb new workers in the labor force and keep unemployment from rising.

Earlier this month, the government reported that the gross national product, the value of the nation's total output of goods and services, rose 2.7 percent in the third quarter of 1984, after adjusting for inflation.

DRI expects the economy to grow 2.5 percent in '85, vs. 6.8 percent this year. As a result, the firm predicts a 7.6 percent jobless rate by the end of next year, vs. an expected 7.2 percent rate at the end of this one.

With disarray in the Organization of Petroleum Exporting Countries leading many economists to predict somewhat lower oil prices in the months ahead, the inflation rate is seen moving ahead only modestly next year.

''We expect inflation to remain fairly low. The consumer price index will run about 4.3 percent (in 1985), with oil prices helping a great deal,'' says Bernard Markstein III, senior economist at Chase Econometrics, a forecasting firm. Last week the government reported that consumer prices rose at a seasonally adjusted 4.2 percent annual rate in the first nine months of this year.

Lowered expectations for inflation are one reason interest rates have been falling. Interest rates normally include a return to lenders for the use of their money as well as a premium designed to offset the eroding effects of inflation on the payments the lender receives.

On Friday, four major banks announced they were chopping their prime, or benchmark, lending rate to 12 percent. The banks were Morgan Guaranty Trust Company, Chase Manhattan, First National Bank of Chicago, and Bankers Trust Company. Bankers Trust dropped from 12.25 percent, the other banks from 12.5 percent.

Slower economic growth, and thus lower demand for credit at banks, also played a key role in the move, economists say.

''There is no question the reduction we have seen in interest rates is tied directly to the slowdown in the economy,'' says Ben E. Laden, chief economist at T. Rowe Price Associates Inc. ''The Fed has changed its attitude toward what is going on in the economy.''

As a result, monetary-policymakers now are less worried about inflation and more worried about recession. Mr. Laden sees the prime slipping to 11.5 percent by year's end.

The recent decline in interest rates is the result of two distinct moves to easier credit conditions, Chase's Mr. Markstein says. One move occurred in the August-September period, he says, and the latest in the last several weeks.

The Fed, however, discloses its policy moves only on a delayed basis. So there still is considerable dispute over the course of Fed policy.

For example, Citibank vice-president Alan Murray says he sees further easing by the Fed, which would bring down the rate on federal funds, the money banks lend each other overnight. Fed actions affect the federal funds rate, which is a major factor in determining banks' cost of funds and thus in determining what banks charge for the money they lend.

Mr. Murray sees the fed funds rate moving as low as 9.0 percent in the short run. Mr. Laden of T. Rowe Price says he expects the federal funds market to tighten a bit, to 10 percent. On Friday the rate was 9.5 percent, down from 11.5 percent in mid-September.

There is no question that the prospect of slipping oil prices, which would help reduce inflation, gives the Fed greater maneuvering room for lowering interest rates.

But the Fed's freedom to lower interest rates is still limited by federal deficits. Most economists outside the Reagan administration believe deficits, by pushing up the demand for funds, also push up interest rates. Late Thursday the government announced the deficit for fiscal year 1984 was $175.34 billion, huge in historical terms, but smaller than 1983's $195.35 billion in red ink.