President Reagan's speech on El Salvador formally opens what could yet become a useful policy collaboration with Congress. To get something near the $110 million in aid he is asking, Mr. Reagan has reaffirmed that he will not send American soldiers or "Americanize the war with a lot of U.S. combat advisers." But he still needs to be persuaded to go further than he has in supporting political talks aimed at ending the war.

The argument for more aid is persuasive. As long as the United States stays committed to influencing the outcome in Salvador, it must pay a certain price. Few members of Congress seem prepared simply to accept the results of a free fall. The argument is over the amount of aid and, more than that, the terms. How good does the Salvadoran government's performance on rights and reforms have to be? How can the aid lever best be manipulated to achieve the desired results? An aroused Congress can strengthen the hand of a president who is ready, as Mr. Reagan now indicates he may be, to bring more pressure to bear.

Obviously, however, the chief condition Mr. Reagan's critics wish him to attach is that the Salvadoran government vigorously pursue talks. The president fell short of that standard in his speech on Thursday. He observed that the United States already supports negotiations among states of the region to stop the flow of arms, respect borders and remove foreign military advisers. But such negotiations, unquestionably desirable, will almost certainly take too long to start--let alone to achieve results--to provide the relief that the shaky American enterprise in El Salvador requires.

No, what the Congress and the critics want, and should want, is movement toward talks between the competing Salvadoran forces. Mr. Reagan rightly opposes talks that would merely "divide up power behind the people's back." He did speak on Thursday for "negotiations within nations aimed at expanding participation in democratic institutions, at getting all parties to participate in free, nonviolent elections." This is fine in theory, but what does he mean in practice?

Mr. Reagan speaks as though a democratic train were already running in El Salvador and the guerrillas should lay down their guns and climb aboard. But there is not so much a train as a train schedule, not much more, despite the elections (to a "constituent assembly") of last year and the onset of reforms. Power is in the hands of armed groups, some uniformed, some disciplined, some respectful of civilians and some not. The powers that be cannot protect even the people who submit to their authority and beg for their protection. To imagine that the potentially reachable part of the opposition will come out of the mountains and out of exile and join a process run by the armed forces is absurd.

The challenge, in brief, is to devise procedures and guarantees that will encourage opposition groups so minded to join the search for a common democratic political solution. The support available within El Salvador for this approach is, we think, far broader than Mr. Reagan appears to have been told. But only the skeleton of such an idea is evident in the administration's pronouncements so far. It needs to be fleshed out, with the assistance of the United States' Latin friends, who will support no other approach. By promoting talks in El Salvador, it should be noted, Mr. Reagan improves chances of widening support for the excellent and necessary collateral idea that talks should also be organized in Nicaragua, between the Sandinista regime and its democratic rivals.

Congress, meanwhile, needs to understand that it is only by offering El Salvador continuing aid that the United States can fairly expect to earn a hearing for the political option. If the aid plug is pulled, that option goes down the drain. The two, aid and talks, comprise the bargain that must now be struck.