As much as constancy's name is woman, the name of the Mideast is political instability. Or so goes the cliche. In fact, most of the major rulers in this part of the world have all sat on top of the heap for a decade or more.

Anwar Sadat of Egypt has been president since 1970; Hafez Assad of Syria since 1971. Saddam Hussein, though he became president of Iraq only last July, has actually been the strong man there since 1974. Yasser Arafat has sat astride the turbulent Palestine Liberation Organization since 1969. Saudi Arabia managed a transition after the assassination of King Faisal in 1975 without even breathing hard. Muammar Qaddafi in Libya goes back to 1969, and King Hussein of Jordan has sat on his throne for 27 years.

Relative success in the 1973 war against Israel explains some of the durability. Sadat and Assad increased their prestige enormously, and are still drawing down the capitol. Some of the luster rubbed off on other leaders.

The enormous rise in oil revenues also figures largely. The billions accruing to Libya, Iraq, Saudi Arabia and the sheikdoms of Persian Gulf buy off direct opposition. At least some of the money has been spread to bolster weaker economies in Egypt, Syria and Jordan.

Then there is the style of government. Most of the rulers in these parts are only once removed -- in fact -- from being soldiers and cops. Even under pressure, they can rely on an effective security apparatus.

Pressurs, abundant to start, are much on the rise now. Great discrepancies of income characterize most of the Arab countries. Corruption is pervasive. So are rivalries between various religious and ethnic communities.

Organized troublemakers, ready to set matches to the gasoline, swarm in the Mideast. The Islamic fundamentalists, who swept the shah from power in Iran, have counterparts in every Arab country. Communists and other radicals generally work hand-in-hand with the religious militants. Though Arafat seems less and less prone to violence and subversion, various branches of the PLO still go in for dirty work against Arab regimes.

Syria, which I have just visited, provides a focal point for all forms of the dissidence. Militant Moslems have for over a year been waging a campaign of assassination and violence against the Alawite minority that runs the country through Assad. A splinter communist group has joined them. In the cities of Hama and Aleppo, merchants and professional groups have staged protests against the regime in Damascus.

Assad first tried conciliation, but the troubles continued, and it now seems clear that he cannot end the difficulty by concessions.

Two weeks ago he initiated a crackdown. Special Alawite security units instituted tough house-to-house searches in Hama and Aleppo. According to the minister of information, Achmed Iskander, arms caches were discovered. He claims troublemakers -- including some West Europeans -- were arrested and ae being held for trial. But other reports, circulating in Lebanon, say the security forces encountered stiff resistance. It is not clear that Assad can save himself by repression any more than by a policy of concessions.

Still he endures. He showed his nationalist credentials last week by joining with Libya, South Yemen, Algeria and the PLO in a meeting of the Steadfastness (or, as some call it, sorehead) Front against the Camp David agreements. Russia gives economic and military aid, and the promise of protection. It is hard to see how anyone can oust him, unless someone manages to win over his security forces.

To some degree, Assad's situation is that of all other leaders in the area. They live on borrowed time -- "on probation," as Iraq's president, Saddam Hussein, once put it. But given the bleak prospects for peace in the Mideast, even that is something.

For apart from the Arabs, the main forces in the Mideast are immobilized. Israel is not going to make a big move until after Prime Minister Menachem Begin leaves office. The United States cannot act decisively until -- at the very least -- the presidential election is over. The Europeans cannot act independently of the United States except in the context of detente with Russia. But there is no serious chance for accommodation with Moscow while Afghanistan burns.

So the world has to mark time on the Mideast. The resilience of the present Arab regimes holds out a ray of hope -- the hope that they can continue without the kind of blowup that would precipitate a political and economic crisis of world-wide proportions.