The testing time is at hand for Ronald Reagan, not just in one area on one issue, but on all of the major pieces of his program at once.

On the economic front, there are clear signs of congressional reluctance to carry through the second round of budget cuts. There are strong hints from prominent GOP legislators that they might like to postpone some of the scheduled tax reductions in order to relieve budget pressures.

In the national defense arena, there is sharp criticism of his decisions on the MX missile basing plan, on the start-up of B1 bomber production and on the overall commitment of funds. The criticism is coming from many fronts, including fellow conservatives, Democratic defense specialists and even some senior uniformed officers.

On the diplomatic front, there is stubborn resistance in Congress to his plan for the sale of sophisticated AWACS aircraft to Saudi Arabia; a growing risk of direct U.S. military involvement in the Mideast in the wake of Sadat's assassination; demands for the United States to take a tougher line toward Israel; and, at the same time, heightened Israeli sensitivity to American efforts to cultivate support in Egypt and Saudi Arabia.

There are growing signs of political discord in the European countries, especially Germany, about U.S. nuclear policy, and disagreement with Canada and Mexico over bilateral issues and the stance the United States is taking toward Western Hemisphere and Third World economic development.

Complicating all this is the steadily worsening and ever-more-public infighting among the president's national security aides.

In this kind of situation, a president can expect to be lobbied vigorously by those who urge him to be "pragmatic" and those who urge him to "stick to your guns."

There is a lot to be said, in almost all these matters, for taking a second look at the positions the president is defending. It is not at all hard to make a case that the tax cuts are excessive, that the MX in hardened silos is a hugely expensive and very short-term expedient, that the B1 is more of a bummer than a bomber against Soviet air defenses.

It is easy to say, now, that the United States and Saudi Arabia both might have been better off had the AWACS surveillance been handled for a few years longer by American planes and American crews. It is equally plausible to argue that we need to be more sensitive to the sentiments of our neighbors and our European allies.

And a lot of folks, at high levels in Reagan's official family, make no bones about the fact that they would like to start with a clean slate in filling the top jobs in the Pentagon, the State Department and the National Security Council.

But conceding all that, the lesson of history is quite clear: at this stage of his term, a president needs, more than anything else, to show his steadiness, his purposefulness and his perseverance in the face of pressure.

And that means, on most of these matters, Reagan is going to have to play out the hand he has dealt--not try to pick up his cards and reshuffle them.

The danger in doing anything else could be described to him by the predecessor presidents he entertained briefly at the White House last week, before sending them to represent him and the country at the ceremonies in Cairo.

Even before he was sworn in, Jimmy Carter wobbled on a key appointment, backing off his first choice for director of the Central Intelligence Agency. In his first nine months as president, he scrapped a key piece of his own economic program. Both actions damaged his credibility with Congress enormously.

Jerry Ford did something the country never expected by pardoning Richard Nixon. And then he did a 180-degree turn on his own anti-inflation program. He never really recovered from those steps.

Nixon was more persevering and successful for a time, but the twists and turns of his Watergate evasions finally destroyed his backing, even within his own party.

What all this suggests is that Reagan may have the option of second thoughts on one or two of the major policy and personnel decisions that are being challenged today. But if he tries to cut and run on several of them, he will be worse off by far than by trying to stay the course. His gamble has to be that his initial judgments were not as wrong as the growing band of critics suggests.