The daily reports from El Salvador suggest that by Salvadoran standards, the situation is relatively stable. That is to say, the killing by terrorism, assassinations and guerrilla skirmishing continues at about the usual rate of 10,000 deaths a year. (On a per capita basis, that would be a half-million deaths in the United States.)

But the so-called central government, a coalition of Christian Democrats and the military, remains perilously threatened by extremists from the far right and the far left. Having recently put down a massive leftist offensive, it is incapable of wiping out the leftist guerrilla forces. Economic chaos enhances the prospects of a move from the right.

In short, El Salvador has all the necessary ingredients for an early test of just about everything the Reagan administration has been saying about its general approach to East-West competition in the Third World -- about international terrorism and human rights, right-wing authoritarians and left-wing totalitarians.

"El Salvador would be a tempting place for the new administration to try to demonstrate a difference," says one old hand from the Carter administration who played an important part in the making of U.S. policy in El Salvador in recent years.

For the departed administration, El Salvador was a proving ground of its approach to Third World conflict. Support for the Christian Democrat/military coalition was fine-tuned to encourage "moderates" against extremists on both sides, with human rights the crucial test. Economic aid was generous, and military aid more modest; both were tied to internal reforms -- land redistribution, in particular.

Now comes the Reagan administration, bent on "replacing" human rights with restraint of international terrorism as its top priority. It argues that "moderately repressive" authoritarian regimes are to be welcomed ahead of even more repressive totalitarian regimes under the influence of international communism.

How much difference would this make? So far, the Reagan administration has not been precise, although Secretary of State Alexander Haig has suggested that he would continue the U.S. policy of support for the present government of El Salvador. Far from abandoning aid, he has said that "as a matter of fact, it may go just the other way."

If even more aid is what's intended, it becomes instructive to examine just how far the Carter administration had gone -- and hadn't gone -- before it laid the El Salvador question aside in the final, frantic days of its term.

On Jan. 10, the far left forces in El Salvador launched a massive offensive, openly proclaimed as an effort to bring the central government down. In early December, the United States had suspended both economic and military help to El Salvador, in protest of the murder of four American churchwomen -- and the government's reluctance to investigate the crime.

Economic aid had soon been restored to head off what seemed to be an impending economic collapse. But not until the Saturday night before the inauguration did the Carter administration hasten to restore military aid, including for the first time "lethal" items, which is to say guns. At the same time, six transport helicopters were rushed to the scene.

A day earlier, I'm told, the Carter high councils had given serious thought to even more drastic emergency help. According to an official in a position to know, national security adviser Zbigniew Brzezinski floated the idea of providing American "mercenaries," if necessary, to fly the helicopters. There was even some talk of authorizing American military advisers to accompany the Salvadorans on operations against the left-wing insurgents, and perhaps to carry arms.

President Carter vetoed both ideas, and some insist their prospects were never very high. "The Vietnam analogy was too close," says one official. Adds another: "It was Reagan's decision to make."

But the things Jimmy Carter didn't do are illustrative, nevertheless, of the road the new administration might find itself on, if more aid to the present Salvadoran government is what it has in mind. That's not the only course available if things go badly in El Salvador -- nor is it necessarily the one the Reagan administration would take. It could always, of course, throw in with the extreme right.

In any case, given where matters were left to Jimmy Carter, any effort by President Reagan to "demonstrate a difference" with his predecessor in El Salvador would require some substantial changes in policy -- and some substantial risks.