When Republican economist Murray L. Weidenbaum showed up at the Joint Economic Committee last week to explain and push the Reagan economic program, he was welcomed by Democratic Congressman Henry Reuss with open arms. Weidenbaum, chairman of the Council of Economic Advisers, "is an old friend and respected adviser" who could be counted on to exhibit "common sense," Reuss declaimed.

But Weidenbaum, wise to the ways of Washington, was not about to allow Reuss to put too much distance between him and his colleagues in the administration, who Reuss asserted were indulging in supply-side and monetarist "fetishes."

Without a moment's hesitation, he told Reuss, "At this point we are all monetarists, and we are all supply-siders. I say that as an eclectic economist who has learned from both and who looks for the truth wherever it lies."

He added that "I'm fond of reminding my keynesian and supply-side friends of the teachings of the great neo-classical economist Alfred Marshall, who taught us quite properly that there are two blades to the economic scissors, supply and demand."

Nonetheless, although liberal Democrats complain privately that Weidenbaum is less experienced in macroeconomics than many of his predecessors on the CEA, he is widely accepted as a more traditional economist and less of an ideologue than many others in the Reagan administration. Even those who would rather have seen someone else in the CEA role tend to agree that Murray is one hell of a nice guy."

When Weidenbaum left Washington in 1971 to go back to Washington University in St. Louis, he had established a reputation as an independent, modest professor who never had been infected with Potomac Fever. He rode the bus to work, shunning a Treasury car because, he said, "I didn't want to get used to it."

Weidenbaum never pretended that his two years in the Nixon administration were "years of uninterrupted joy or success." But he insists he never had to say anything, publicly or privately that he didn't believe in. The other side of the coin was that he never publicly attacked the policies of the Nixon administration.

Since 1975, he has been the director of the Center for the Study of American Business at Washington University. There he developed a reputation as a hard-liner on deregulation, a subject close to the heart of all true Reaganites, and therefore destined to be an important part of the CEA's work in the Weidenbaum regime.

The mild-mannered Weidenbaum was a late-comer to the Reagan team. He didn't sign on as head of Reagan's regulatory reform task force until his first choice for the GOP nomination, John Connally (his final Treasury boss), dropped out.

Weidenbaum was the last of the key economic appointments made by Reagan. It actually didn't happen until a couple of days after the inauguration, and the Senate didn't confirm him until last Tuesday. The search is still on for the other two CEA members.

To some, Reagan's delay in nominating a council chairman suggested a downgrading of the council, as well as a put-down of economists in general. In an interview at the Executive Building a few days ago, Weidenbaum said he thought neither concern has validity.

He said he has been assured that the CEA will have a key role in the evolution of Reaganomics, and the White House has taken pains to give him high visibility. It appears to outsiders that Weidenbaum may prove to be a crucial balancing force between the sometimes contradictory goals and statements of Reagan monetarists on the one hand, and Reagan supplysiders on the other.

Weidenbaum indicates economic policy-making is somewhat less structured now than in the Carter administration, when an Economic Policy Group under the Treasury secretary was supposed to coordinate affairs. An equivalent group has not yet gotten under way in the Reagan White House.

Instead, Weidenbaum, Budget Director David A. Stockman and Treasury Secretary Donald Regan meet as a troika with the president and other White House officials on general economic policy. With Stockman and presidential aide Martin Anderson, Weidenbaum is included on the overall Budget Review Committee. Other Cabinet members are involved in a separate set of Cabinet councils dealing with specific issues.

Weidenbaum said he is comfortable with the Reagan program, which he says is "feasible, useful and can easily be defended." In fact, this is what he has been doing on virtually a nonstop basis along with Stockman and Reagan on radio, television, in breakfast and luncheon interviews and in congressional appearances.

He brushes aside the argument that the compromise figures that support the Reagan program are largely guesswork. "As a veteran of 30 years' experience in economic forecasting, I have never depended entirely on any computer run," he said. "A good deal of judgment has to go into the final results."

He once said that those addicted to a "rigid all-purpose cure for our economic ailments have done us a great disservice. At times, they remind me of the old Steve Allen TVroutine -- 'Here is the answer, now what is the question?'"

Relying on 3-by-5 cards on which he has written the key facts and statistics, Weidenbaum is less dynamic an outside salesman than the super-confident Stockman. He is more earnest, a bit more professorial and less given to cosmic rhetoric."At first," says a Treasury official, "Murray sort of 'underwhelms' you. But he always outperforms the initial impression."

On the inside, administration sources indicate he is holding his own with the other Reagan stars. He had a voice in the decision that it would be politically unfeasible to lower the top tax rate on unearned income from 70 percent to 50 percent all in one year and a hand in trimming academic supply-side language from the president's economic message to Congress This earned him a cut-slash from one team of columnists, presumably fed by some purists inside the administration.

Although he won't talk about it, Weidenbaum also appears to have been instrumented in shooting down the original and highly optimistic set of projections on the economy that had been prepared in Stockman's office by John Rutledge and Lawrence Kudlow, two of the more ardent supply-siders.

Weidenbaum admits that there is much to do to complete the Reagan program. Multibillions in budget reductions for late years have yet to be spelled out, for example. Even the basic benefits of the Social Security System -- now untouched as part of the "safety net" -- will have to be looked at, he confides.

Way back in June 1970, when he was an assistant Treasury secretary in the Nixon administration, Weidenbaum had indicated sympathy for government intervention to curb excessive wage and price increases through a form of "incomes policy" (as had former Federal Reserve chairman Arthur F. Burns at that time). Weidenbaum was promptly disavowed by then secretary David Kennedy.

Weidenbaum has come a long way since then to more conventional Republican views, or he wouldn't be a member of the Reagan team. With a smile, Weidenbaum shrugs off the 1970 episode as an aberration, a youthful indiscretion. "Live and learn," he says today.