Congressman urge switch in US Salvadoran policy

Some 100 members of Congress, mostly Republicans, have signed a letter urging President Reagan to switch policy and support negotiations between El Salvador's government and leftist guerrilla insurgents.

''The escalating crises in El Salvador, Guatemala, and Nicaragua are reaching a critical juncture and run the risk of involving the United States in a major regional conflagration,'' the letter says.

It is the strongest show of disagreement in Congress so far with the President's military support for the Salvadoran junta. It urges Mr. Reagan to accept Mexican President Jose Lopez Portillo's offer to mediate in negotiations between the factions in El Salvador, including the guerrillas.