If violence-ridden El Salvador manages to avoid an all-out explosion between extremists of the right and left, a lot of the credit will go to U.S. Ambassador Robert E. White.

A veteran diplomat at 53, the plain-spoken White made his career in Latin America and his reputation in human rights advocacy. He was probably the best choice President Carter could have made last March for the unenviable position of the man in the middle in El Salvador.

White's assignment was to make clear to every element in El Salvador -- right, left and center -- that for once the United States is determined to support middle-of-the-road moderates. It's an overdue switch from Uncle Sam's traditional backing of tinhorn tyrants whose only claim on our allegiance is their devout anticommunism.

Because it is such a drastic change in policy, it has aroused outrage from the "betrayed" right, and disbelief from the suspicious left.

White has become the lightning rod for the extremists' wrath. "The situation is dangerous for me because I'm a symbol of the United States," White admitted philosophically to my reporter Katharine Koch.

After only four months in El Salvador, White already has the distinction of being the only "gringo" on the hit list of 250 individuals to be "physically eliminated" by the right-wing secret Anti-Communist Army. The list also includes leftists, members of the junta, members of the intelligentsia and religious, labor and professional leaders.

White's residence was surrounded by right-wing demonstrators for an entire weekend in May. He sat tight till Monday morning, when the Marines used tear gas to disperse the mob. The American Embassy is a fortress described by one visitor as "better guarded than the vault in Fort Knox."

White's enemies are not just Salvadoran extremists. After a reception he gave for the U.S. citizens there, one U.S. businessman was overheard muttering, "I'd like to kill him." Undeterred by the hostility White coolly informed the local chamber of commerce, "As strongly as we reject a solution of the extreme left, we reject the right. We want a democratic solution."

The rightists see the U.S.-backed junta's agrarian and banking reforms as "railroading socialism into El Salvador." The leftists say the land reform was "made in U.S.A." from the old dictatorship's blueprints, and suspect the CIA may be behind it. In fact, one of the first things White demanded -- and got -- when he moved into his Salvadoran hot seat was the recall of the CIA station chief.