To travel from Egypt to Israel, as I have just done, is to move from one unreality to another. Politically, at least, Egypt lives in the sunny world of the daydream. Israel inhabits the dark realm of the nightmare, and the United States ought to keep the disparity in mind as it tries to negotiate a settlement between the two countries.

Hope bubbles strong in Egypt in large part because conditions are so bad. The terrible poverty of the countryside and the universal dilapidation of Cairo and other cities leaves nowhere to go but up. President Sadat, a skillful leader and one trusted after eight years in office, has repeatedly sounded the theme that peace means change. Change, he need hardly add, means prosperity.

So virtually everybody - 99.99 percent is the figure on almost all lips - backs Sadat in his moves toward peace. Even those with doubts acknowledge they are spitting against the wind. "We have to wait until the peace and prosperity euphoria is over before we can speak out," one former senior diplomat who asked that I not mention his name told me.

To be sure, other Arab countries, including such supposedly moderate states as Saudi Arabia and Jordan, have been critical of the deal cut by Sadat at Camp David on the ground that it is a separate Israeli-Egyptian peace that leaves vague a settlement on Jerusalem and the future status of the Palestinians. But Sadat himself repeatedly insults King Hussein of Jordan as (in the words he used to me the other day) "someone who will turn anywhere for money." He did not bother to consult the Saudis about his trip to Jerusalem last year or his posture at Camp David.

If Sadat treats the other Arabs with so much contempt, it is because he knows that in the end they will go along with him. It seems hardly the office of the United States, in these circumstances, to run after the Jordanians and Saudis with intimations of better deals on Jerusalem and the Palestinians. Not only do the Saudis and the Jordanians reject the U.S. appeal but, worse still, the American stand only causes Sadat to up - as he has just done - his demands.

For a variety of reasons, the Israelis in general, and Prime Minister Menachem Begin in particular, are far less resilient. Mr. Begin himself is relatively new to power, and he heads a fragile coalition of even more inexperienced ministers. The government's handling of the economy alone - with inflation now running at 50 percent annually - invites a suspicion of total incompetence.

The agreement the prime minister brought home from Camp David has - from the Israeli point of view - intrinsic risks. He yields major security bases in the Sinai peninsula - notably the Rafiah salient along the coast. Its acceptance of autonomy for the area west of the Jordan River is seen by most Israelis as a handle that will enable the Arabs to establish a Palestine state within five years.

Even worse in Israeli eyes than the character of the accord itself is the route Begin took to get it. He came to office as a known hawk, and he kept reassuring Israelis in the strongest terms - and by symbolic stands on such matters as settlements - that he was standing firm in his original beliefs.

But time after time, on the vital as distinct from the symbolic points, he has made concessions. On June 18, for example, he said that he would never discuss the future sovereignty over the west bank of the Jordan. On July 25 he gave Foreign Minister Moshe Dayan authority - later incorporated into the Camp David agreements - to put the question of sovereignty over the West Bank up for grabs at the end of a five year interim period.

The upshot is a widespread suspicion of Begin himself. At least a dozen people in different positions and with very different attitudes toward a settlement have said to me in the last few days that "We don't know whether Begin is deceiving us or deceiving himself."

In these conditions opposition to further concessions has grown apace. Every component part of the government coalition opposes new concessions. The diehard right-wingers who have broken with Begin - notably Schmuel Katz of the prime minister's own Herut Party - are more than ever against it. So is the dovish, left wing of the Labor Party opposition.

Putting new pressures on Israel at this time, accordingly, makes little sense. The parts of the agreement that make for pressure - the parts relative to Jerusalem and the Palestinians - cannot be immediately operative anyway. Sadat does not need concessions on these items to carry his country. So for the time being, at least, the issues of Jerusalem and the Palestinians are secondary. What is primary and immediate - and what the Carter administration ought to concentrate on almost exclusively - is the Sinai accord between Israel and Egypt.