United States Secretary of State George Shultz went off to Central America this week partly because there was something useful he might be able to do there. But it was also partly because, for the time being, there is little he dare do about his other big problem - the Middle East.
He cannot do anything about the Middle East because Israel is in the throes of a political crisis. The government of Yitzhak Shamir can fall at any moment. Little useful diplomatic business can be done with Israel or about the Middle East until the present political uncertainty is resolved in Israel.
But there is still time to save, or to try to save, what has become a degenerating condition in El Salvador.
In that unhappy country, three years of US military aid to the government seems further from achieving its goal than when it started. The goal was to build a ''military shield'' behind which a right-wing tyranny could be converted into a reformed, moderate democracy capable of attracting general popular support.
The shield has of late been dented and penetrated. Rebel forces have been able for months now to exercise the military initiative in several parts of the country. Even the best US-trained government units seem to be unable to make headway against the highly mobile and resourceful rebels. Defections from government forces to the rebels are reported to be frequent. Rebels claim that they obtain most of their weapons and ammunition by capture or purchase from government units.
Behind this less-than-impenetrable shield, the infamous ''death squads'' have apparently continued their work largely unrestrained by repeated US warnings. Vice-President George Bush was there Dec. 11, repeating the warning with all possible earnestness and solemnity. But according to the Roman Catholic Church and other independent sources, the ''death squads'' continued their accustomed nocturnal rounds.
This week Secretary Shultz again repeated the warnings that the ''death squads'' must be curbed if US aid is to continue. He used the phrase ''death squads'' at a formal dinner given him in San Salvador by officials of the El Salvador government. That government itself is presumably doing what it can to end the right-wing terror in its midst. The trouble, of course, is that the leaders of the government are less powerful than the unofficial junta of conservative landowners who control the terror by night.
The El Salvador government has been able to reassign a few officers believed to be linked to the ''death squads,'' but Mr. Shultz would hardly have gone down there in person if Washington was satisfied that the condition had been cured. The trouble, for Mr. Shultz, is that continuation of the deadly terror on the right undermines congressional willingness to provide more funds.
What most worries the administration is that if conditions continue to deteriorate in El Salvador, the only way to save the country from falling to the rebels would then be direct US intervention. That would not be popular in Congress or at home generally. It could become a political liability to President Reagan in the election campaign.
The fragility of the Shamir government in Israel was exposed during the previous week when, on Jan. 25, the government squeaked through a no-confidence vote with a majority of 62 to 56. That margin was made possible by a political deal that held three shaky votes on the government side. There is no certainty that they can be held longer.
Next evidence of weakness came on Jan. 29 with the resignation from the cabinet of Mordechai Ben Porat, a minister without portfolio, who has joined two other members of the present ruling coalition in saying he is ready to vote for dissolution and an early election.
The indication is that Prime Minister Yitzhak Shamir will find himself outvoted on the next nonconfidence motion unless he can rebuild his coalition on a stronger basis.
The opposition now counts 59 votes for dissolution. Outside observers think the opposition will probably be able to force either an election or a national government.
Behind the political crisis is the dreadful state of the Israeli economy, which is plagued by nearly 200 percent inflation plus the financial burden of maintaining an army of occupation in southern Lebanon at a cost of around $1 million a day. The plain fact is that Israel is financially and emotionally overextended. The cost of the occupation of southern Lebanon in money and lives, plus the cost of attempted annexation of the occupied territories, is proving too much for the Shamir government.
Anything that Washington might do or say to Israel at this moment could trigger a resolution of the political situation in Israel, but not necessarily for the better from Washington's point of view. The only wise course is the one Mr. Shultz pursued this week. He went to El Salvador, while having the President's special assistant for the Middle East, Donald Rumsfeld, continue his rounds of countries there.
Obviously, the long-term purpose of the Rumsfeld wanderings is to try to put together an ad hoc grouping of the so-called moderate Arabs which could authorize King Hussein of Jordan to open peace talks with Israel, if and when there is a government in Israel capable of entering into such negotiations.
That project was aided by the readmission of Egypt to the Islamic Conference Organization. This made respectable, in Arab eyes, the meetings President Mubarak of Egypt has been having with the PLO's Yasser Arafat. These meeting are aimed, among other things, at reviving a license for King Hussein to negotiate with Israel toward a general peace.
It is conceivable that this new and changed situation in the Middle East could become the context of the first serious drive for a general peace in the area since Camp David. From the standpoint of domestic politics and the economy, Israel is weaker now than at any time since its earliest days. It needs peace with its Arab neighbors as never before. More Israelis than ever before are in favor of peace. American diplomacy has not yet been able to persuade Israel to spend the occupied territories for a general peace. What Washington's persuasion could not do, triple-digit inflation might.