Washington — Almost everyone in Washington, including top Republican leaders in Congress, now warns President Reagan that without policy changes, the US economy soon will be in very rough waters.
''Here is what we face,'' says one of the top economists in the government, who asked not to be named. ''The economy will get worse before it gets better.
''Then, in mid-1982, recovery will start. Inflation and interest rates will be lower than they are now, but unemployment will be very high.''
The jobless rate, now 8.4 percent of the work force, will climb above 9 percent, possibly to 10 percent, before the recession ends, many analysts believe.
''The budget deficit then becomes scary,'' the economist says, ''not so much for 1982 as for '83 and '84, when recovery is fueling the demand for credit.''
This is because private business and individuals will crowd the capital market for funds just as the US Treasury - forced to finance deficits well above
''In that situation,'' the official says, ''either the Fed (Federal Reserve Board) tightens up on the money supply, in which case interest rates shoot up, or the Fed loosens the reins and inflation climbs.''
Sen. Bob Dole (R) of Kansas, chairman of the Senate Finance Committee, says that he and other top GOP leaders - including the chairmen of the Budget and Appropriations committees - told the President of the ''political realities'' he faces in the coming election year, unless the galloping deficits are tamed.
Senate Budget Committee chairman Pete V. Domenici (R) of New Mexico fleshed out the warning by forecasting steadily rising deficits in fiscal '83 and '84, based on the President's current tax and spending policies.
These men and their House counterparts are the people on whom Mr. Reagan must depend to get the next round of spending cuts through Congress. Republican leaders tell him there is no way that enough can be trimmed from nondefense and nonentitlement programs to stem the spreading pool of red ink.
They - like the economist just cited - insist that policy changes are needed to avoid real trouble. What changes?
They are easy to sum up, but hard to accomplish: major tax increases, substantial cuts in defense, and scaled-down entitlement programs, such as social security, medicare, and the like.
Defense, entitlements, and interest on the national debt swallow 75 percent of the total federal budget, leaving 25 percent in ''discretionary'' programs to confront the budgetary carving knife.
Embraced in that 25 percent are public housing, job training, welfare programs for women, children, and the aged - all of which already were slashed in last summer's round of budget-cutting.
''Tax increases and cuts in social security and entitlements are politically very painful,'' says a high government source, ''when people still are thinking in terms of recession and high unemployment. Especially,'' the source says, ''because the necessary legislation would have to be passed before the November '82 elections, to be effective for 1983.''
Beyond that, Reagan shows no visible sign of softening his opposition to major tax hikes nor any inclination to cut defense outlays.
In his latest press conference Reagan said flatly that he plans no new tax increases. Aides amended that to mean no increases beyond $22 billion worth of ''revenue enhancement'' for the Treasury, to be achieved largely through closing some tax loopholes.
The President rejects out of hand any postponement of the personal income tax cuts, 20 percent in all, due to come July 1, 1982, and July 1 of the following year.
Given the climate of uncertainty hanging over Eastern Europe and the Middle East, there seems little likelihood of meaningful defense cuts soon.
That leaves entitlements, on which - at least so far as social security is concerned - no real action now is contemplated by Congress or the White House until after next year's congressional elections.
By that time, a newly appointed bipartisan committee chaired by economist Alan Greenspan will have made a report to the government on what steps should be taken to sustain the social security system.
Despite the President's distaste for tax hikes, that is the area on which debate is likely to center in coming weeks, perhaps months. White House advisers tell Reagan major tax boosts are needed to prevent huge deficits from overwhelming his program, politically and economically.
''The President's program still has a chance to work,'' says a prominent Democrat, ''if he and his supply-side believers can be brought to accept some kinds of tax increases, other than individual income taxes.''
Such increases - whether in the form of gasoline tax, oil import fee, or excise taxes - will shrink the amount of overall tax relief voters had hoped to get. But they may be the only way to save Reagan's program as a whole.