Washington — Over the months since President Reagan took office, his advisers made - and still make - two claims about his now-beleaguered economic program: * The President's three-year, across-the-board tax-cut program emulates the ''supply side'' tax cuts inaugurated by President Kennedy in the early 1960s.
* Wall Street, or the investment community, should have had the good sense and vision to support the Reagan program by allowing interest rates to drop.
Walter W. Heller, an architect of the Kennedy tax cuts, knocks the props out from under the first claim, by saying that Mr. Reagan's approach in fact reverses what the Kennedy administration did.
''We put the supply-side first,'' says Dr. Heller in a telephone interview, speaking of the Kennedy years, ''and the demand side last.''
First, under the Kennedy program, business was given tax breaks, notably the investment tax credit - still in force - and liberalized depreciation rules.
These 1961-62 measures, says Heller, were designed to give businessmen an incentive to invest in new plant and equipment - in short, the supply-side approach.
Not until 1964, Dr. Heller adds - after the supply-side tax cuts had begun their work - were personal income tax cuts (the demand side of the equation) put in place.
Under the Reagan program, personal and business tax cuts were passed at the same time, with the benefits weighted toward the demand, or consumption, side of the scale.
''The Reagan tax-cut program,'' says Peter G. Peterson, former secretary of commerce to President Nixon, ''was enormously large and targeted more toward consumption than savings.''
Mr. Peterson, chairman of Lehman Brothers Inc., is a leading international investment banker. Dr. Heller, regents professor of economics at the University of Minnesota, is one of the nation's most respected liberal economists.
Both said in separate interviews that the construction of the Reagan tax-cut program is a major flaw in the President's economic approach, leading among other things to deep doubts among investors.
''The investment community,'' Mr. Peterson says, ''looking ahead, foresaw huge (government) deficits building up, especially because, as part of the tax-cut program, the three-year, across-the-board cuts were to be followed by the indexation of taxes.''
Beginning in 1985 taxpayers no longer will be propelled into higher tax brackets and hence higher taxes by inflation. However welcome this may be to individual taxpayers, it will shrink US Treasury revenues by $34 billion the first year, in Peterson's estimate.
This implies higher budget deficits, which in turn leads investors to hedge their bets about future rates of interest and inflation.
Dr. Heller cites inflation as another reason why a direct comparison between the Kennedy and Reagan tax programs is, in his view, invalid.
At the time of the Kennedy tax cuts, he said, ''We had had four years of price stability, with inflation rising at 1.5 percent a year.
''As a result, monetary and fiscal policy could work hand in hand - a stimulative fiscal (tax and budget) policy and an accommodative monetary policy.''
In other words, the Federal Reserve Board could - in a period of low inflation - afford to increase the money supply to accommodate a growth in public demand for goods and services.
''Today,'' Heller says, ''fiscal and monetary policy are at each other like scorpions in a bottle.''
Peterson puts the same concern in different words: ''After the recession,'' he said, ''I foresee a clash - a contradiction of policy - between the economic growth rates anticipated by the tax-cut program and the tight money policy of the Fed.''
One such consequence of such a clash, widely forecast by analysts outside the Reagan administration, would be higher inflation and interest rates, as the US Treasury, businessmen, and families all scrambled to borrow money.
Unless the mix of President Reagan's program is changed - perhaps by giving less money to the Pentagon and raising taxes to shrink soaring budget deficits - that goal of investor confidence may elude the White House grasp.