Washington — All the ingredients are present for the rumor mills of Washington to start grinding away:
A first-term Republican President desperately needs lower interest rates to validate his economic policies and give his party a boost in the November election.
Suddenly, a few months before voters go to the polls, interest rates begin to tumble. The Federal Reserve Board nudges the process along by lowering the discount rate and injecting reserves into the banking system.
The stock market responds eagerly, chalking up record gains and volume. Bond prices soar.
Ergo, cry the rumor mills, Ronald Reagan laid his full-court press on Paul A. Volcker, chairman of the Federal Reserve Board, and the Fed caved in, deserting its longstanding policy of keeping money tight.
Sounds logical, but that isn't the way it really happened, many experts claim. ''Events are doing 80 percent of the work to bring interest rates down, '' says Brookings Institution economist Charles L. Schultze, '' and the Fed perhaps 20 percent.''
Among these events:
* After exceeding the Fed's targets for the first half of the year, growth of the money supply has markedly slowed, giving Mr. Volcker and company some room to ease credit.
* The economy is weaker than people had expected and is likely to experience only a mild recovery at best in 1982. This indicates that business borrowing may be slack this fall.
This is bad news in a general sense. But it does mean that the huge borrowing needs of the US Treasury - an estimated $100 billion in the last half of 1982 - may not collide with heavy private sector credit demands.
* Passage of the $98.3 billion three-year tax increase package signals the federal government's determination to begin controlling budget deficits.
All this does not indicate that the economy itself will show immediate improvement. The rapid fall off in interest rates, in fact, stems more from the weakness of the economy than from any other factor.
Unemployment may top 10 percent, some analysts claim, before interest rates fall to the point where the housing, automobile, and other durable goods industries begin to recover.
Opinion also is unanimous that next year Congress and the White House together nust make a renewed effort to reduce budgetary red ink, either through fresh spending cuts, another tax increase, or both.
Almost certainly, spending cuts will have to include curbs on the growth rate of social security and other benefit programs if significant progress is to be made on reducing federal deficits.
Fed officials, meanwhile, have agonized in recent monthsover what high interest rates were doing to the economy. They were acutely aware that countless small businessmen, farmers, savings and loan institutions, banks, and cash-starved corporations, were at the ragged edge of disaster.
During the early months of 1982, however, the 12-member Federal Open Market Committee (FOMC), which sets monetary policy for the Fed, elected not to ease credit.
''For reasons not yet clear,'' says Fed governor J. Charles Partee, ''business demand for credit, or loans, remained high - higher than expected.''
''The indicated need for outside financing was down sharply,'' says Partee, given the fact that business activity was dormant. ''But the actual demand for money fell only moderately.''
Fed officials concluded that many businesses were so short of cash that they were borrowing short term - not to invest in expansion or new investment, but simply to finance current operations.
In the face of this, and knowing that US Treasury borrowing was on the increase, the FOMC - composed of the seven Fed governors and five presidents of regional federal reserve banks - chose not to risk renewed inflation by loosening up on credit.
During the first half of 1982 the money supply, especially the key measurement called M1, rose faster than the FOMC considered prudent. Swift growth of the money supply breeds inflation.
But then, over the last six weeks or so, business demand for capital has notably slackened. M1 has dropped within its 1982 target range. The Fed, with less inflationary risk, could and did encourage interest rates to slide down.
This it did by injecting more reserves, or loanable money, into the commercial banking system and by dropping the discount rate - the interest the Fed charges to member banks - in three steps from 12 to 10.5 percent.
This set off a chain reaction of decline among other short-term rates, with the prime rate - which banks charge to their best corporate customers - falling from 16 to 13.5 percent since mid-July.
No one denies that there was, during this developing process, tremendous political pressure on the Federal Reserve Board to help bring interest rates down.
This came both from Republicans and Democrats and was tied only partly to the election, said a Fed official. Basically, the pressure reflected concern, shared by the FOMC, that any worsening of the economy - triggered perhaps by the collapse of banks or the bankruptcy of nationally known firms - could lead to panic.
''You don't,'' says economist Barry Bosworth, ''put the economy through a ringer in an election year.''