SPARE A MOMENT for unhappy El Salvador, last seen sinking into its 11th year of civil war. The United States has taken a potentially useful step by voluntarily extending a cut in military aid to the government to encourage peace negotiations. A response by anti-aid congressional Democrats could make a difference.

Last year Congress halved military aid to El Salvador in justifiable disgust at the armed forces' murder (and coverup) of the Jesuit priests. But it also necessarily provided for restoration of the cut if the FMLN guerrillas exploited it. This the FMLN did, opening an offensive, killing civilians and importing aircraft-killing missiles. The White House could have just turned the aid back on full. Instead, to encourage the FMLN to take "a serious and constructive approach" to United Nations-sponsored cease-fire negotiations, it suspended delivery for 60 days, until March 15. A cease-fire would clear a political space for municipal and national assembly elections scheduled at that time.

Among government and guerrillas, there is a consensus that what the U.N. negotiator calls "a wide array of political agreements" is the condition for a cease-fire. That puts a burden on both sides to negotiate expeditiously and in good faith. Naturally there is disagreement on this matter. But it is incontestable that the guerrillas, in devising their strategy, have sought to play on American fatigue and division. The earlier aid cut could only have encouraged them along this path.

Congressional Democrats must face these political facts. If their purpose is to substitute negotiations for war, then they should use the 60 days to press the guerrillas no less than they press the government and armed forces. They should be as vigorous in calling upon Salvadoran guerrillas to accept a political settlement as they were in calling upon Nicaraguan guerrillas -- the contras.

Further, the Democrats should be no less eager to cut off the FMLN's foreign-arms supplies now than they were to cut off the contras' then. The FMLN's Soviet-made missiles, including ones that downed the American military aircraft whose surviving crew were murdered by guerrillas a few weeks ago, came from Nicaragua. Under Soviet as well as American urging, the Sandinista defense minister, Humberto Ortega, has coughed up a few freelance competitors in gun-running, but he has yet to take himself unequivocally out of the trade. Perhaps some of the Democrats could send him a message, too.