The Reagan administration, shrugging off sharp criticism from Democrats, said yesterday it is sending an extra $55 million in emergency military aid to El Salvador. It said continuing U.S. aid is necessary to save that country from a "probable victory" by communist-led insurgents.

Assistant Secretary of State Thomas O. Enders told congressional committees the $55 million is required to stave off a guerrilla campaign to thwart the country's elections and declared: "To withhold . . . assistance at this point would be to abandon El Salvador."

His testimony was greeted with skepticism by House Democrats who questioned sending ever-increasing sums of money to assist a civilian-military junta that they charged continues to tolerate brutal repressions of human rights. Rep. Clarence D. Long (D-Md.), chairman of a House Appropriations subcommittee, told Enders he regards El Salvador as a "bottomless pit."

The additional $55 million that the administration intends to send immediately would bring to about $81 million the total military aid for El Salvador this year, more than double last year's $35 million. In addition, Enders indicated that the administration will propose "in the order of magnitude" of $100 million in economic aid for El Salvador as part of a new Central American or Caribbean assistance plan it is readying.

Although many Democrats were angry at the decision to rush in a new aid infusion amid new reports of killings of civilians by the military, there appeared to be nothing they could do to block it. The $55 million will come from an emergency fund controlled by the president who is required only to tell Congress how he spends it.

The rest of this year's aid to El Salvador comes from a regular foreign aid appropriation that the administration became free to spend after it certified to Congress last week that the country is making progress in the area of human rights.

There was talk among Democrats of passing some sort of rescission resolution to strip the administration of some foreign aid funds to show their displeasure but they found no way to block the actual transfer of military equipment included in the $55 million.

Enders' visit to advise congressional committees was complicated by disclosures yesterday that El Salvador forces had slain 19 civilians during a raid on a San Salvador slum area. The government claimed it was pursuing subversives and that some members of the government patrol had been injured.

Enders seemed to contradict that version yesterday when he told the House subcommittee that he finds it "difficult to buy the notion that there was a fire fight." He was also called on to explain the Salvadoran government's role in a major shootout last December in Morazan Province where natives claimed soldiers killed hundreds of citizens.

Enders quoted from a new U.S. Embassy report that said the involvement of government soldiers in that round of slayings could not be proved or disproved. It also said the insurgents who fought government forces had done nothing to get civilian noncombatants out of the way of the encounter.

Enders also said he believes the reports of the number of deaths to be exaggerated. In a report from the scene, The Washington Post reported evidence of several hundred civilians dead.

Some Democrats have talked of passing legislation to cut off all aid to El Salvador because of the latest atrocity reports.

Enders told a Senate Foreign Relations subcommittee that that course would be disastrous. Terminating aid would be a "massive blow" to the government, would lead to even more bloodshed, and would "probably" result in an insurgent victory, he testified.

The winners, he said, would follow the Nicaragua pattern, which he described as a brief interlude of political pluralism followed by a one-party state.

Of the $55 million in new military assistance, about $25 million will go to replace Salvadoran aircraft destroyed in a raid on a government air base. Enders said replacement of those aircraft was urgently needed to help the government fend off guerrillas who are stepping up offensives in advance of the March election.