Perhaps no one steps into a tougher job at a tougher time than economist Martin Feldstein, who later this month will take over as chairman of President Reagan's Council of Economic Advisers after the resignation of Murray Weidenbaum. Feldstein comes on board as the nation sinks deeper into recession, with economic policy-making in chaos.

The three-member CEA is probably at its lowest ebb since 1953, when Congress stalled on renewing its budget, creating a brief hiatus in the life of the council at the start of the Eisenhower administration.

Although Weidenbaum says that he's had "ample opportunity" to present his advice to Reagan, it is clear that the economist from St. Louis has not had the influence on or rapport with the president enjoyed by some of his more aggressive predecessors.

In fact, the White House decided to go forward with a mid-year economic review this year based on projections that Weidenbaum found too optimistic to endorse.

And while Weidenbaum's decision to return to his academic pursuits at Washington University preceded the internal debate on the mid-year forecast, the fact that the White House was willing to adopt economic projections with which its CEA chairman could not agree is tell-tale evidence that the CEA has dipped to a low point in the pecking order.

"There is no question that if Murray had more clout -- if they listened to him more -- he would still be there," says an influential White House adviser.

Weidenbaum can be direct when he wants to be -- for example, he is among the administration's most forthright antiprotectionist spokesmen. But on many issues, he appears to waffle. "Murray hedges all the time, and he doesn't come across" clearly, says another economist. "Yet, I'm sure that in his own eyes, he perceives himself as forceful, and battling the dragon more than is really the case."

Here is a fairly typical Weidenbaum formulation, from his July 23 press conference on his resignation: "I'm as enthusiastic today as I was in January 1981 about the policy of the Reagan economic program." That allows for an interpretative judgment about how "enthusiastic" he was last January. And in any event, he mentioned "the policy" of the program, not necessarily the program itself.

In one critical episode where Weidenbaum left no doubt about where he stood, it proved to be costly: for about a week last summer, Reagan stopped talking to him, after he told the president that the federal deficit would increase, and that the country faced a serious recession. This happened at the end of July, just after the huge tax cut had just passed, and it obviously was not what the president wanted to hear.

In hindsight, it should have been clear at the start that Weidenbaum would head into trouble because he was neither a supply-side extremist nor monetarist ideologue. His presumed qualifications for the council chairmanship stemmed from his expertise in the field of deregulation. Indeed, when he was asked at a press conference to say where his advice had been taken, he cited, primarily, progress on regulatory reform.

What may have hurt Weidenbaum most was that he came late to the program-shaping process: Reagan had already made a commitment to the huge tax cut and to a monetarist approach at the Federal Reserve Board. As a moderate, Weidenbaum was able to negotiate an adjustment in some of the wilder predictions proposed by supply-side extremists, so that the initial economic forecast made better sense (or less nonsense) than it would have otherwise. "He fended off the crazies," acknowledges a leading Republican.

But it was a damage-limitation exercise: Weidenbaum eventually reaped the worst of both worlds: he was suspected by the "true-believers" among Reaganauts of being a closet Keynesian. On the other hand, those old-line Republicans who worry most about the huge deficits, and thus believe in a stricter fiscal orthodoxy, didn't perceive him to be of much help (although Weidenbaum sees himself as "a hawk on budget cuts").

Initially, it may be remembered, Weidenbaum did not forcefully contradict the startling contention of his colleague William Niskanen last year that huge budget deficits don't matter. It took a prod from the White House to get both himself and Niskanen off an embarrassing limb.

To be sure, Weidenbaum had to grapple with what any economist working for Reagan -- including his successor -- would face: this particular president, even more than most, doesn't have a high regard for the economics profession. For a long time in Reagan's career, most economists were Keynesians, and that has helped formulate in his mind a distrust of all economists.

Surely, it is not Weidenbaum's fault that after 18 months of this administration, the country encounters recession instead of the economic revival promised by the president. More than most of his colleagues, Weidenbaum perceived what was going on. But there was nothing much he could do about it. Doubtless he will be happier back in St. Louis.

A shift in the top CEA spot doesn't translate into a panacea for the nation, which has deeply rooted financial and economic problems. It looks as if huge deficits will be encountered for a long time to come, whether or not the new tax-increase approach carries in Congress.

So Feldstein, an academic from Harvard and president of the National Bureau of Economic Research, deserves full marks for being gutsy. He lacks experience in the Washington political trenches, and as Treasury Secretary Donald T. Regan would probably tell him, that can be a handicap for a while. Feldstein would probably also concede that he is not the best forecaster in the world -- and forecasting is an important part of the CEA's job.

But Feldstein -- an authentic supply-sider (that is to say, not a "crazy") -- is held by most of his peers (Republican and Democrat alike) to be not only a broader economist than Weidenbaum, but potentially more effective. He has many innovative -- if controversial -- ideas, especially in the field of Social Security. What success he will have in dealing with the California Mafia in the White House ultimately will depend on how he and the president hit it off.