Everyone's No. 1 question: what will the President do?

* A federal deficit of $400 billion over the next three years.

* Deadlock and stalemate between the separated branches of government including a new factor, the quasi-independent Federal Reserve System.

* A temporarily improving economy over the next half-year, followed by a possible relapse into recession, with attendent unemployment and even inflation.

The foregoing are the possibilities - real or imagined - that have produced one of the most searching examinations of the American economy, the budget, and also the relationship of White House and Congress in recent years. What's President Reagan going to do is the question everybody is asking as a possible way out of the situation is sought.

Few times have shown such tension and uncertainty over the course of government policy in the economic realm. The anxiety has spread to the financial markets where the chief Wall Street indicator has had a long descent from a high on the Dow Jones industrial average of 1024 in mid-1981 to a present level around or below 800. Various conservative financial groups that supported Mr. Reagan's election have expressed concern.

Most recently, a poll commissioned by the American Stock Exchange indicated that support for the President's economic program among Wall Street professionals has dwindled from 67 percent a year ago to 41 percent now. The survey found that the size of the federal deficit is the investment community's biggest complaint. It's most mentioned solution is a trim in proposed defense spending.

The arms budget is questioned in Congress, too. The enlarged Reagan defense bill for the next five years would be $1.5 trillion. According to Otto Eckstein, chairman of Data Resources Inc., testifying to the Senate Budget Committee this month, this is ''a 17 percent-a-year increase in military spending for the four years'' and that this comes after Congress has passed ''the largest tax cuts in American history.'' He argues that the defense buildup is too big to be accomplished even if attempted.

The administration proposed, and Congress enacted, a 10 percent cut in individual income tax rates to take effect next year. Some congressmen now call this too big.

Mr. Reagan argues that there are ''early signs of recovery'' and cites a drop in the rate of inflation and a recent increase in the rate of personal savings.

Speaking to a meeting of the National Association of Manufacturers here, he urged them to stand firm behind his program and to stop ''haggling'' in their support.

''What we need now,'' he said, ''is not last-minute haggling or displays of blatant self-interest - we need the support that only America's businessmen can give us. We need you to participate in our task forces on private sector initiatives and government cost-cutting, but most of all we need you to get on with the business of economic recovery, to look for imaginative ways to invest and grow and to provide jobs for the unemployed.

''Let me be honest with you . . . I've been a little disappointed lately with some in the business community who have forgotten that feeding more dollars to government is like feeding a stray pup. It just follows you home and sits on your doorstep asking for more.''

Nearly everybody agrees that the deficit prospect of the 1983 Reagan budget - originally indicated at $96.4 billion but now estimated at $121 billion or larger - is unacceptably high, but Congress waits for specific modification from the White House. Half a dozen compromise alternatives are proposed in Congress including one by Senate Budget Committee chairman Pete Domenici (R) of New Mexico. Another comes from a middle-roader, Sen. Ernest Hollings (D) of South Carolina of the Appropriation and the Budget committees.