The State Department is asking members of Congress and administration officials to postpone all but crucial travel to El Salvador until at least late this month because of increased worry that they could become victims of political violence.

One member of a group whose trip was canceled, who asked that his name not be used, said he had been told that the State Department feared a possible resurgence in rightist death-squad activity after the U.S. elections.

He said El Salvador's rightists "have always thought the U.S. interest in human rights would end Nov. 6," assuming that President Reagan is reelected.

In the first three years of his administration, Reagan consistently opposed congressional efforts to attach human-rights conditions to military and economic assistance for El Salvador.

Although Reagan denounced rights violations, Salvadoran conservatives made no secret that they did not believe he was serious until he sent Vice President Bush to El Salvador in December to demand a halt to death-squad murders and threats.

Death-squad murders plummeted after Bush's visit and have not returned to previous levels, although they have not stopped.

The Los Angeles Times reported the new restrictions on Friday.

Official State Department guidance on the tightened travel rules said a general travel advisory has been in effect in El Salvador for at least three years, warning tourists that the country is undergoing political unrest.

Officials said the restrictions have been tightened at least three times in the last three years, most recently after rightist death threats against Ambassador Thomas R. Pickering.

State Department security for Pickering was increased after those threats and again when the threats were repeated in late September, according to Salvadoran diplomatic sources.

The current tightening of travel rules follows a spate of public threats against Salvadoran President Jose Napoleon Duarte from the Maximilian Hernandez Brigade, a terrorist group opposed to Duarte's talks with leaders of the guerrilla left.

"Essential" travel is not affected by the new policy, and a congressional staff group planning to evaluate U.S. Embassy security in El Salvador and elsewhere in the region later this month has not changed its travel plans.

But at least one official of the Agency for International Development, John Kelley, has been unable to travel to El Salvador since last summer after receiving telephone threats because of his work helping to organize last May's presidential elections.

Other officials contemplating travel to El Salvador said the State Department had expressed concern to them about possible new leftist guerrilla violence, especially bombing attacks in major Salvadoran cities, in addition to concern over rightist violence.

The leftist guerrillas, who have been trying to overthrow the government since 1979, have been increasingly on the defensive in the countryside and have previously launched attacks in the capital.

The head of U.S. Embassy security in El Salvador, a Salvadoran citizen, was shot and killed a block from the embassy two weeks ago.

A leftist group claimed responsibility.

In its official guidance, the State Department referred to "a temporary cutback in nonessential travel to El Salvador" that would last as long as necessary.

The statement said the restrictions resulted from worldwide assessments of embassy security arrangements, made over the past two weeks in the wake of repeated terrorist violence at U.S. facilities in Lebanon.