At any time, economic forecasting is a chancy business, as economists and their clients -- public and private -- have found out in recent years. And the process is even riskier this year at a time of great uncertainty generated by two unknowns: the outcome of tax "reform" legislation, and the shape of a Gramm- Rudman compromise on the budget deficit.

The tax bill drafted by Democrats in the House would benefit low-and middle-income families and, therefore, may help sustain consumer spending. But most businessmen fear that the bill, which raises corporate taxes, would hinder investment and possibly exacerbate their problems of competing with more efficient Japanese industries.

Meanwhile, the Gramm-Rudman budget-balancing proposal would cause a drop in both defense and nondefense government spending in order to meet the automatic deficit reduction targets. James C. Miller III, President Reagan's new budget director, let it slip that he expects Reagan to propose a $144 billion deficit for fiscal 1987 (the Gramm-Rudman target), "whether or not" Gramm-Rudman becomes law.

Whether such a slash could in reality take place is another question. In an appearance at the American Enterprise Institute, Miller himself noted that the $85 billion saving over three years projected in the congressional budget resolution earlier this year now appears to have dwindled to a mere $7.5 billion.

Nonetheless, many forces are beginning to converge, pointing to a gradual reduction of the deficit, including the easing of world tensions in the aftermath of the Reagan-Gorbachev summit. And Miller's willingness to cite a $144 billion deficit target for fiscal 1987 suggests that some sort of tax increase or "revenue enhancement" has not been completely written off, despite Reagan's well-publicized dislike for taxes.

For these and other reasons, most economists aren't looking to a zippy year in 1986. Mellon Bank's chief economist and senior vice president, Norman Robertson, told the AEI conference that the U.S. economy might grow by 2.5 to 3 percent, a respectable performance but not enough to cut the unemployment rate. Some others come in with a figure under 2 percent.

Slow growth in the United States would be bad news for the rest of the world, which has depended heavily on a booming U.S. economy to soak up its exports. It would be especially grim for Third World countries. They already are suffering from declining prices for commodities and raw materials that account for most of their export earnings.

As usual, the net results in the year ahead will reflect a balancing of positives and negatives. The good news is that business has few inventory problems. And despite the decline of the dollar, which doubtless will push up the cost of Japanese cars and other imports, the country is expected to experience a low rate of inflation. Interest rates, encouraged by an accommodating Paul Volcker, will stay down or go lower.

Treasury Secretary James A. Baker's initiative on the dollar is a Reagan policy reversal designed to deflate protectionism in Congress. The initiative is expected to help turn the trade deficit around beginning late in 1986.

But even if steel, chemical and other manufacturing industries get some help from a cheaper dollar and stronger yen, basic weaknesses won't be solved overnight. As Roger E. Brinner and Robert A. Gough Jr. of Data Resources, Inc., point out, competition is becoming tougher "in those areas that the United States has long dominated," particularly agriculture and high-tech products.

Also on the negative side, Rob only the Gramm-Rudman bill (which he believes is unworkable), but the fact that 1986 would be the fourth year of the expansion cycle. That's when things are bound to wind down.

Consumers are the driving force behind the recent upsurge. They have been spending almost every nickel they have left after taxes. Thus, the savings rate has dwinded to an all-time low of 2.7 percent. History tells us it won't stay that low -- which means that spending is sure to slow down.

Meanwhile, financial markets are under strain, and the office building boom has already resulted in a bust that threatens to be even more acute in 1986. The banking-S&L system is also stretched thin, overexposed not only in real estate but in farm loans and in the Third World.

The bottom line, according to Mellon Bank's Robertson: "The economy begins to look a little fragile for 1986, and truly vulnerable to a setback in 1987."