Democratic and Republican congressional leaders emerged from a briefing yesterday saying they are convinced that the Reagan administration has "hard evidence" that outside communist forces are supplying weapons to the leftist guerrillas in El Salvador's civil war, and they promised bipartisan backing for the administration's support of the embattled government there.

The attitude of the congressional leaders, who were briefed by Secretary of State Alexander M. Haig Jr., was summed up by Sen. Charles H. Percy (R-Ill.), chairman of the Senate Foreign Relations Committee, who said:

"I think that those outside forces should be on notice that this nation will do whatever is necessary to prevent a communist takeover in El Salvador . . . . We will not stand idly by while outside forces -- outside our hemisphere or within our hemisphere -- are feeding arms to one particular faction attempting to bring down what looks to be a centrist government."

These promises of congressional support came as the State Department, in its strongest statements so far on the El Salvador situation, said it intends to support that country' civilian-military government despite charges it has abetted murders by rightist forces, and without linking the support to political reforms or progress toward solving the murders of four American Roman Catholic missionaries.

Department spokesman William Dyess, making clear that the administration considers external aid to communist insurgents the major threat to U.S. interests in Central America, said:

"Our principal concern has been that a sitting government has been challenged by an insurgency that has been supported from outside, specifically by Cuba through large-scale smuggling of arms. We are concerned both for that government and for the precedent that would be set if we did not act."

Although the resumption of military aid to El Salvador was ordered by then-President Carter, his administration had linked the aid to an inquiry into the death of the women missionaries and a more effective crackdown by Salvadoran authorities on human rights violations. However, Dyess, echoing Haig's priority emphasis on combatting international terrorism, stressed:

"As far as I know, there are no such conditions now. What is primarily of concern now is that government use the aid and use it effectively . . . . Our principal concern is for the survival of that government and that it will be in a position to deal with the military situation."

Dyess refused to confirm that three teams of State Department officials are in Europe and Latin America to inform other governments of the evidence the United States has collected about Cuban aid to the guerrillas -- even though the visits have been announced by various foreign governments. Although Percy said the administration in planning to issue a "white paper" detailing its evidence, Dyess also refused to comment on that.

Reliable sources said the administration's reticence in discussing the oversea missions was due to the request of some governments that do not want the visits publicized because of strong support in their countries for the Salvadoran leftists.

The sources said official reluctance to discuss the "white paper" was caused, in part, by a desire to see how much effect U.S. pressure has in getting the government of Nicaragua, through which most of the arms allegedly are moving, to cut off the flow. Under conditions laid down by Congress, a public charge by the administration that Nicaragua is aiding the Salvadoran guerrillas would require cutting off all U.S. aid to Nicaragua, and the administration believes that such a step would be counterproductive in drying up the arms flow.

For that reason, the sources said, publication of the "white paper," which tentatively had been expected this week, may be delayed until next week to allow more time for assessing how cooperative the Nicaraguans are being.

Percy said that use of American military troops in El Salvador "would be highly unlikely" but added that increased U.S. military and economic aid "is in the cards." The administration has concluded that such increases almost certainly will be required, but it has not yet decided on the size and precise nature of additional assistance.