The road to peace in the Middle East was opened by the strong qualities of President Anwar Sadat and Prime Minister Menahem Begin. Now their weak points are beginning to show, along with those of King Hussein of Jordan. The upshot is that the road to peace has become thick with obstacles.

Begin's attitudes are the old virtues - dignity, principles and strong convictions. At a time when Zionism was on the defensive around the world, he was a natural rallying point for the Israeli people. He still commands the popular support necessary to make the concessions any peace would entail.

But as a gentleman of the old school, trained in a Prussian-style Gymnasium and at the law school of Warsaw University, Begin is not a natural negotiator. He attaches overwhelming importance to words and formulas. He does not see them as starting points in a process. Indeed, he seems to think of bargaining as beneath his dignity.

So he has stuck hard and fast on the principle of Jewish settlements in both the Sinai, which will eventually be turned over to Egypt, and the West Bank of the Jordan, which probably will become the core of a Palestinian entity with self-rule under a sovereign to be decided in the future. Similarly, he has balked, as if he were being asked to swallow rat poison, at any mention of "self-determination" for the Palestinians on the West Bank.

Sadat's great quality is the capacity for sweeping dramatic moves that transform historic perspectives. He showed it first in 1972 by unilaterally expelling most Soviet advisers from Egypt. Again in 1973 when he launched the October war. Most recently with the visit to Jerusalem in November.

Sadat has a weakness for the histrionic. He cannot resist the starring role. He loves posturing before the television cameras. It is a rare day when he does not give an interview to some newspaper or magazine, and, like most persons concerned with effect, Sadat goes for the ringing phrase and has little patience with detail.

He initiated the present difficulty in Egyptian-Israeli relations by saying flatly to the magazine October that Egypt would not tolerate any Jewish settlements in the Sinai, and he rammed the point home by saying that the Israelis should "burn" the settlements. He keeps using the term "self-determination" in the loosest way. In an impromptu television interview the other day he said, "Anything that links the Palestinians and self-determination sounds good to me."

As to King Hussein, he is a quintessential survivor. He has stood up despite threatened coups inside Jordan; despite the loss of half his country to Israel in the Six-Day War of 1967; despite a bitter guerrilla struggle with Palestinian nationalists in 1970; and despite repudiation by other Arab leaders who, at the Rabat summit meeting of 1974, designated the Palestine Liberation Organization - not Hussein - as the sole valid negotiator of Palestinian interests.

At present King Hussein could play a truly helpful role in moving negotiations along. By having his representatives join the talks now going forward in Cairo and Jerusalem, the king could give the lie to charges that the Israelis and Egyptians were making a unilateral deal. Moreover, Hussein could also smooth the handling of the difficult West Bank problem since the ideal solution is a Palestinian entity linked in some federation with Jordan.

But the king is gun-shy, and since the death of his wife last year, he seems dispirited. He has been slow to assert his interest - which is actually very great - in getting back territory from the Israelis. Instead of joining the negotiations, he has sat on the fence, waiting to see what turned up, and looking nervously over this shoulders for clues that don't come from the leaders of Saudi Arabia.

In consequence there has been a slowdown of events, a loss of the momentum originally generated by Sadat's visit to Jerusalem. Disagreements that should have been kept below the surface, or settled quickly, have become sticking points.

A difficult, marathon negotiation shapes up around finding ways to give the Palestinians a homeland that does not threaten peace in the area. While the outlook remains basically good, it is not because solutions are in view. It is chiefly because Sadat and Begin have invested so much capital in the peace initiative that neither can afford to fail.