President Reagan, refreshed from his spectacular banquet-level visit to China , turned his attention this week to the most important piece of unfinished business on his election-year agenda - the rescue of El Salvador.
On Tuesday he summoned congressional leaders to his office for a preliminary sermon on the importance of saving ''democracy'' in El Salvador and fending off ''grim consequences'' to United States ''security.'' He repeated the sermon in equally strong terms later in the day to a forum of businessmen.
Then on Wednesday evening he went on national television with a full public performance, illustrated with charts and studded with detail, about danger to US strategic sea lanes and ''to our own southern border'' should Cuba and Nicaragua succeed in overthrowing ''democracy'' in El Salvador.
It was a major push for getting from Congress the money Mr. Reagan wants for the present regime in El Salvador. The chances that he will get it are much improved by his histrionic skill and also by the apparent results of last Sunday's Salvadorean election.
The election results have not yet been officially reported, but Mr. Reagan told reporters he had seen projections that ''Duarte is the winner and by a sizable majority,'' which he said is ''pleasing to us.'' It was pleasing because the presumed victory of moderate Jose Napoleon Duarte over rightist Roberto d'Aubuisson would ease the passage of the money bills through Congress.
One measure of Mr. Reagan's confidence that he can top his earlier success against Central American ''communism'' on the island of Grenada by a second win in El Salvador was that news of Moscow's boycott of the Los Angeles Olympic Games this summer caused scarcely a ripple of reaction at the White House.
That news was a disappointment in the athletic community, but probably has negligible political effect. It was widely interpreted in news reports as a Soviet reprisal for former President Jimmy Carter's US boycott of the 1980 Olympic Games in Moscow. So if political blame there be, it is Mr. Carter's, not Mr. Reagan's. Besides, probably many voters are just as glad the Soviets are not coming to the games.
The White House said it would not make any special effort to try to persuade the Soviets to change their mind about the Olympics. In diplomatic quarters, the cancellation was seen as a symptom of a presumed decision in Moscow to avoid doing business with Mr. Reagan during this election year.
It would help Mr. Reagan's political image if the Soviets would resume talks with him about nuclear weapons, but all indications seem to be that they know it would help him and are determined therefore, if for no other reason, to keep aloof.
That means that Mr. Reagan must campaign as a warrior against ''communism'' rather than as a peacemaker. Since it has to be so, he is obviously set to play out that line to the limit in Central America, where the odds for success are better than they were in Lebanon, and seem to have lengthened in his favor in recent days.
The presumed Duarte win in El Salvador means that US aid will go to a country headed by a declared moderate who stands for all the ''right things,'' such as an end to ''death squads,'' revival of land reform, a political amnesty, and attempted peace talks with Nicaragua.
Whether Mr. Duarte as president of El Salvador could move down such a road depends, of course, on the colonels and generals who receive the guns from the Pentagon and command the troops trained by American instructors. The colonels and generals hold the real power in El Salvador. The Salvadorean president can do only what they allow him to do. His life is at their pleasure.
So far, they have allowed Duarte to live, and presumably to win the election, and they seem likely to allow him to take office. The further possibility is that they will even allow him to revive land reform and get credit for a decline in the work of the ''death squads.''
Representatives from the military leadership in El Salvador are frequently in Washington. Recently, they have been not only in the Pentagon but also on Capitol Hill, where they have discussed reforms in their own behavior. Such reforms are necessary to overcome congressional reluctance to Mr. Reagan's proposals for more guns.
Promises of improved behavior have reportedly been made to congressional leaders. If fulfilled, there will be fewer corpses found along the highways and byways of El Salvador. Actual decline in ''death squad'' work will mean less resistance in Congress to Mr. Reagan's proposals. As of this week, the chances are that Congress will provide the requested funds.
Beyond that, the prospects are speculative. But Mr. Reagan's firm commitment to the existing regime in El Salvador will itself encourage the regime to continue the fight. Add to this the presence of US naval vessels patrolling the waters off El Salvador and US reconnaissance planes flying over the country. Add the presence of substantial US forces in Honduras. Add also reports of a decline of confidence in Nicaragua.
Many of the experts on the Middle East could see, and did predict, the ultimate failure of the Reagan attempt to shape the future in Lebanon. Experts tend to see things differently on Central America. The place itself is deep inside the military frontiers of the United States. Some think Washington can't lose if it puts its full strength into any operation it attempts in the area.
The limiting question about the El Salvador operation is the willingness of Congress, and public opinion, to support it. As of this weekend it is a plausible assumption that Congress will provide the funds and public opinion will permit the use of them to the degree sufficient to save the present anticommunist regime in El Salvador, at least through the US national elections in November.
It is even possible that Nicaragua will decide to come to terms with Washington and abandon its support of Salvadorean rebels. Mr. Reagan would then go before the voters in November as the man who saved Central America from Moscow.