The recent deaths of 75 Nicaraguan children in a fiery helicopter crash have become the focal point of an increasingly bitter Sandinista campaign against the United States and its support for counterrevolutionary guerrillas.

The official rage reflects growing frustration in the revolutionary leadership at repeated and increasingly large attacks near the border with Honduras. Well-connected diplomats here say it also illustrates how U.S. hostility strengthens hard-line factions in the Sandinista government, fostering more radical policies in a leadership leaning toward Marxism but whose political system is still being defined 3 1/2 years after its victory.

The denunciations of Washington often link President Reagan personally with the children's deaths. One Christmas-season commentary on the official Sandino Radio compared President Reagan to King Herod, who in the Bible ordered babies murdered in an attempt to eliminate a threat to his power from the newborn Christ child.

No evidence has surfaced that the Dec. 9 crash was the result of counterrevolutionary gunfire. The Soviet-built helicopter was transporting 88 women and children from border villages that the Sandinista government said had to be evacuated because of frequent guerrilla attacks. The Defense Ministry did announce that a rescue helicopter later came under machine-gun fire as it approached the burning wreckage.

Armed with these indications, the Sandinista leadership has rejected speculation here that the downed helicopter was simply overloaded. Instead, it is characterizing the crash as an example of what the United States and exile forces on the border are doing to the Nicaraguan people. The first announcement came from Culture Minister Ernesto Cardenal, who interrupted a speech inaugurating a Latin American art exhibit the night of the crash to give the news and declare:

"This is the result of President Reagan's trip to Central America."

The Defense Ministry followed the next day with a straightforward communique describing the deaths. Twenty-four hours later, however, Nicaraguan radios were linked to a national network that broadcast expressions of sorrow and denunciations of Reagan and U.S. policies throughout the day.

Defense Ministry Humberto Ortega told a rally that evening that the 75 children "have given their young lives because of the criminal situation resulting from the unjust war that Yankee imperialism is waging against our young revolution. We could say that the martyred children . . . are victims of the Yankee-Somocista reactionary attack on our revolutionary working people."