As El Salvador's guerrillas put mounting military pressure on the government there, some U.S. officials in the region are expressing concern that the regime is without the leadership and authority to win the war or effectively run the country.

Plans under way to hold presidential elections there before the end of this year, instead of March 1984 as previously announced, are part of an effort to fill this vacuum. Until the question of who is in charge can be resolved, effective action against the guerrillas, or pursuit of a negotiated settlement with them, seem virtually impossible.

Despite growing congressional pressure, the Reagan administration and many members of the Salvadoran government remain fundamentally opposed to negotiations with the Marxist-led guerrillas, whom, they argue, cannot be trusted to abide by any agreement.

Such support for dialogue as has developed on the government side during the past several months is based mainly on a desire to be seen to be exploring all possibilities, and a hope that the more moderate elements of the leftist opposition could be enticed away from the hard-line rebels and into an electoral process they would be unlikely to win.

But given the divisions and weaknesses of the current regime, there is reason to believe that the government would be more likely than its opposition to fall apart in the negotiation process.

A U.S. official in San Salvador today insisted that "the timing of elections is essentially a Salvadoran issue to decide." But, he said, "our position consistently has been that we support the elections at the earliest possible time in order to sort of get this thing wrapped up."

The Reagan administration portrays elections as the only acceptable political solution to the civil war.

In contrast, it rejects the power-sharing negotiations that have been sought by the rebels and their political allies and that have been gaining support in the U.S. Congress.

But with the rebels' confidence bolstered by the successes of their ongoing offensive, and the disparate political forces inside the government more divided than ever, there is less reason now to believe that the election strategy will provide a solution than there was when it was first attempted a year ago with the vote for a constituent assembly.

When last year's elections were planned, with U.S. assistance and encouragement, it appeared likely that the moderate Christian Democrats would win a legislative majority. The assembly would then choose a similarly moderate president, presumably Christian Democratic leader Jose Napoleon Duarte, who would have the authority to override rightist opposition to reforms and effectively run the war. From a position of strength in the polls, the Christian Democrats could also initiate talks with the left, party officials said at the time.

Although the Christian Democrats won a plurality, the assembly ended up dominated by a rightist coalition put together by ex-Army major Roberto D'Aubuisson, and the plan fell apart. Although the coalition voted D'Aubuisson as assembly leader, the Army, under Defense Minister Jose Guillermo Garcia, intervened to keep D'Aubuisson's party from naming a new president. Instead, the Army's favorite, nonpartisan banker Alvaro Magana, was installed.

But Magana had no electoral constituency. Neither did he have complete executive control. In a compromise with the forces represented in the assembly, he named three vice presidents, and divided up his Cabinet, among the three major political parties--D'Aubuisson's Arena Party, the Christian Democrats, and the military-allied National Conciliation Party. Thus, Magana emerged as a kind of board chairman, trying to make the best of a situation that left his government constantly at odds.

Last September, Magana tried to pull together some sort of consensus on broad issues by persuading all the parties to sign a document, called the Apaneca Pact, that was an assemblage of their various campaign promises. At the same time he tried, with some success, to divide Arena's coalition within the assembly and weaken the right's political power.

While Magana appeared to have made some gains toward shoring up his authority as president, at least until the current guerrilla offensive began to gather momentum in the past four months, his efforts have been undermined by the need to find points of agreement among the often bitterly antagonistic parties.

One example of the difficulties he has encountered was the establishment of a "peace commission" called for in the Apaneca Pact. Designated as a quasi-governmental body that would have the freedom to explore a guerrilla amnesty or even, some suggested, possible talks with the left, its formation had been stalled for six months--until Monday--by feuding among the parties over who would be named to it.

Now, according to political analysts in El Salvador, even though there is general agreement on moving up the date of the elections, a date has not been set because each party is trying to avoid taking the lead--a move that would open it to charges of opportunism.

For the guerrillas, the feuding inside the government has been a cost-free bonanza.

"The Farabundo Marti National Liberation Front was capable of defeating the maneuvers of Duarte and the Christian Democrats" when they led an interim government before last year's elections, Bernardo Torres, a senior guerrilla commander in El Salvador's Chalatenango province said recently. "It has defeated the political maneuver of the elections. It is taking apart the 'Government of National Unity' of the Pact of Apaneca. In El Salvador, there are no possibilities of a solution" if the left does not participate.

As civilian regimes have come and gone during the past three years, the one relatively stable element in the Salvadoran government has been the leadership of the Army under Defense Minister Garcia. Yet even that appearance of unity has been severely undermined in recent months, as the war effort has faltered. Gen. Garcia's control was seriously undermined by a mutiny against him by a top field commander in January.

The third branch of the Salvadoran power triumvirate, the year-old constituent assembly, has had its own problems. Under D'Aubuisson's domination, the assembly has assumed the function of a legislature in addition to its responsibility for writing the new constitution, which is due out next month. The assembly has weakened other branches of the government without gaining control itself. As D'Aubuisson's coalition has fallen apart, there is no clear leadership even in the assembly.

Members of its directorate have said that they expect to complete the constitution, under which the new elections would be held, by the end of the month. Many political analysts in El Salvador consider it unlikely that the document emerging from the assembly will differ substantially from the 1962 constitution. Several of its provisions were suspended by a coup in 1979. One observer said it is doubtful that divisive social issues will be directly addressed in the document.