The legislative session to name El Salvador's new provisional president was canceled unexpectedly today by the assembly's rightist leadership, indicating a heightening of tension over division of power.

The constituent assembly directorate, controlled by supporters of former Army Maj. Roberto D'Aubuisson, called the 60 deputies individually around noon to cancel the 3 p.m. session, giving no reason. D'Aubuisson's Nationalist Republican Alliance Party, known as ARENA, its Spanish abbreviation, is fighting to stop the election to the country's highest post of a moderate banker, Alvaro Alfredo Magana, 56. Two members of ARENA indicated that the move was an attempt to stall for time.

Rumors circulated wildly in the absence of any concrete information, and a broadcast on an ARENA-supported radio station said there had been a decision to elect a conservative military man, Col. Nicolas Carranzo, the head of the state telephone company Antel. But the broadcast did not say who had made the decision, and no one could confirm it.

Officially, ARENA leaders charged that the armed forces, which back Magana for the presidency, had threatened the lives of members of another rightist party, the National Conciliation Party, to get them to vote for Magana. National Conciliation, which ruled as representatives of the landed aristocracy for decades, holds the balance of power in the assembly's decision.

The third major party in the drama here, the Christian Democrats, who have said they support Magana, appeared amused by the cancellation.

"They have some problem of a lack of an agreement," said Christian Democratic party secretary Julio Samayoa. With 24 votes in the assembly, the Christian Democrats are a strong minority but cannot exercise control.

National Conciliation deputies were unavailable for comment, but several said earlier today that there was deep division within the party over the way the military had intervened in the political process on Magana's behalf. With U.S. encouragement, the full leadership of the armed services told the parties last week to form a government of national unity, preferably under Magana, or risk losing all U.S. aid.

Implicit in the Army's "very strong suggestion," as one party leader called it, was the threat of direct military intervention to force the warring political parties to come to an agreement. The armed forces have wielded political power for many years in El Salvador, and its highest officials are known to be increasingly impatient with the politicians' inability to form a united front against the continuing guerrilla threat.

The Salvadoran Army unleashed up to 4,000 troops backed by warplanes and helicopters Tuesday in the largest military drive of the year against rebels planning May Day attacks, United Press International quoted military sources as saying.

Deep divisions among the parties were clear yesterday as the rightists pushed through a decree assigning most power in the upcoming provisional government to the constituent assembly, leaving relatively little for the new president. After sharp debate in which the Christian Democrats tried to reserve some power for the presidency, the assembly voted itself authority to ratify all Cabinet members and assistant secretaries, appoint all judges and members of the election commission and to control the budget.

The Christian Democrats succeeded in having a commission appointed to study the question of who should name mayors and local councils, and also managed to defeat a proposal that would have allowed the assembly to legislate whatever it liked during the provisional period, whether or not laws already exist on the subject. But debate was short and the vote was unanimous on that issue, indicating that support for the proposal was minimal.

The assembly also voted to "recognize" existing land and economic reforms, but abolished the decree power that had made them possible under the various military-civilian juntas that have ruled for the past 30 months. The legislators affirmed all the laws passed during that period.

The Christian Democrats were unhappy with the outcome, but noted that the presidency must still have some power because the rightists are fighting very hard to keep it out of Magana's hands.