THE NEW element in the El Salvador tragedy is the growing openness of support flowing to the guerrillas from outside the country. The other day 100 guerrillas of unknown national origin were detected coming across the beach in circumstances redolent of both Nicaraguan and Cuban official sponsorship, and reports of arms supplies from these sources are becoming increasingly persistent and detailed. Whether the change is one of kind or degree is uncertain, since El Salvador's borders have never been airtight. What is clear is that the guerrilla left, intent on exploiting the American transition to surge to power, has set aside a good bit of its usual discretion and is coming on as strong as it can.

Two things are noteworthy about the guerrillas' "final offensive." The first is that, notwithstanding their claim to be fulfilling a popular revolutionary mandate, they are getting little popular support. Part of this no doubt reflects the general antipathy toward violence and, indeed, toward authority in El Salvador -- the government's own popular standing is unimpressive -- but only a part. At the same time, though the government is in general control, the guerrillas have shown a hit-and-run capability that could let them tease and tear the government indefinitely. While the ruling civilian-military junta has held itself open for a political dialogue with some of its leading rivals, moreover, those rivals have not reciprocated. A demon has seized El Salvador. No realistic observer can expect any falling off in the battle soon.

It is suggestive, though perhaps not conclusive, that the "final offensive" was set in motion during a period in which the United States had suspended military aid to the government in the hopes of inducing it to check violence in its own ranks against peasants and their supporters. The onset of the offensive, plus progress reported in investigating that violence, has since led the Carter administration to turn the aid back on. All signs suggest that the Reagan administration will turn it on harder. Military aid is no substitute for the reforms that alone offer the prospect of enduring legitimate rule. But neither is there any avoiding of the need for careful, adequate, effective military aid. Nicaragua and Cuba, and somewhere behind them the Soviet Union, have left Mr. Reagan no choice.