President Reagan's request for increased military aid to El Salvador ran into major new opposition in the Senate yesterday as the administration moved to win over critics by announcing a new effort to make sure that political representatives of the guerrillas can take part in the coming elections there.

Assistant Secretary of State Thomas O. Enders told a Senate subcommittee that the administration will be making "detailed proposals . . . on how to achieve universal participation in the . . . elections."

His testimony came shortly after two influential Democrats, Sen. Daniel K. Inouye (Hawaii), ranking minority member on the Senate Appropriations subcommittee on foreign operations, and Senate Minority Leader Robert C. Byrd (W.Va.), spoke out on the Senate floor against the first $60 million installment of the additional $110 million in military aid Reagan has requested.

Inouye's subcommittee has veto power over Reagan's request, which involves shifting military aid from other countries. The request also could be vetoed by the Senate Foreign Relations Committee, which has scheduled a vote for Thursday, or the House Appropriations subcommittee on foreign operations, which will hear from Secretary of State George P. Shultz Wednesday.

The administration is hoping that efforts to bring the political arm of the Salvadoran guerrillas into the upcoming elections, while carefully avoiding any negotiations over sharing of political power before the elections, will satisfy critics in Congress who fault the president for seeking a military rather than political or diplomatic solution to the conflict there.

Enders testified that "Some in Congress have expressed the hope that negotiations within the framework of democratic institutions can achieve results. We believe that this country and other OAS members can help in this regard. Together we should be able to assist the Salvadoran government to provide the guarantees of personal security, of access to media for campaigning, of a fair count, of respect for the results of the votes cast which all participants are entitled to expect."

Noting that Constituent Assembly President Roberto d'Aubuisson last week called for the political arm of the guerrillas, the Frente Democratico Revolucionario, to take part in the elections, Enders said that a new peace commission had been charged "to undertake the contacts necessary to ensure it.

"Both we and others will be making detailed proposals on how to support this effort to achieve universal participation in the upcoming elections."

While Enders was trying to smooth the waters, another administration witness, Defense Undersecretary Fred C. Ikle, was roiling them by attacking the European allies for their lack of support for U.S. policy in Central America.

Under questioning by Sen. Jesse Helms (R-N.C.), chairman of the subcommittee on Western Hemisphere affairs, Ikle called the European allies' "role-playing" in Central America "very disappointing. They either want to remain quite ignorant about what really goes on and permit themselves to be deceived by totalitarian propaganda or some may be outright mischievous.

"One European ally, France, has supplied arms to Nicaragua. Now other European allies are supplying considerable economic assistance to Nicaragua, but they refuse to help others who want to build up democracies in El Salvador and other Central American countries."

Ikle noted that since the Sandinistas took over Nicaragua they have received $1.6 billion in non-Soviet aid, more than twice the $440 million they have received from the Soviets.

He added that "The vitality of the Atlantic alliance depends on this military thrust in Central America being halted," since the Soviet presence in Cuba and the Caribbean would divert U.S. resources in the event of war.

Inouye's statement on the floor that he would vote against increased military aid to El Salvador was a significant development because the veteran legislator, a decorated war hero, has supported the administration in the past and is influential with his colleagues.

Charging the Salvadoran armed forces with violence and corruption, and comparing the Salvadoran regime with that of former Cuban dictator Fulgencio Batista, Inouye said, "We must ask, are we, in the name of anti-communism, setting the stage for another Castro?"

Echoing the statements of his Democratic counterparts in the House who have called for negotiations with the Christian and Social Democrats within the exiled political arm of the guerrillas, Inouye said, "I believe the solution to the conflict in El Salvador lies within El Salvador. Before any additional military assistance is provided to El Salvador, leaders in the government and the military should agree to engage in negotiations with all parties to the conflict."