The suggestion by Sen. George McGovern (D-S.D) last Monday of international military intervention in Cambodia was greeted with surprise, some anger and widespread questioning of whether the famous foe of the Vietnam war had taken momentary leave of his senses. Yesterday McGovern made clear that he was both serious and determined in his crusade to persuade the world to do something about the reported slaughter in Cambodia. In a Senate speech and an interview, he renewed his call for the United Nations to take "collective action" and explained how and why he came to feel the way he does.

This was "not out of blue," said McGovern, but was something building in him for weeks as the U.S. government and press denounced the trials of two political dissidents in the Soviet Union but nearly ignored "possibly two million people" reported to have died at the hands of the regime in Combodia.

"Do we turn away because of the bitter mistakes we made for so long in Vietnam, which helped to unleash the savagery of Cambodia?" McGovern asked a nearly empty Senate chamber early yesterday. ""Do we turn away because Cambodia is small and weak? Do we turn away because Cambodians are Orientals far from our shores?

"I hate needless and ill-conceived military ventures. That is why I opposed our military intervention against Ho Chi Minch's popularly based revolution for independence in Vietnam. But to hate a needless and foolish intervention that served no good purpose does not give us the excuse to do give us the mass murder in another time and place and under vastly differing circumstances," he declared.

In his office afterward, McGovern said part of his objective was consciousness-raising about Cambodia and part of his strategy was "the old shock technique." I obtaining attention, at least, McGovern succeeded beyond his expectations.

Within an hour or so of his surprising suggestions to State Department witnesses at the Senate Foreign Relations Committee last Monday, dispatches on McGovern's statements were clattering across the teletypes of the Associated Press and United Press International, and his telephones were ringing off the book with inquiries.

Early Tuesday he appeared at length on ABC television's "Good Morning America" and CBS television's "Morning News," and on page one of The Washington Post and many other newspapers. NBC television quickly dispatched a camera crew to interview him, and broadcasts a mindebate on his statements on the "Today" show.

For several days it took three staff aides to answer the telephone, some callers saying it is about time that action was proposed to stop the Combodian bloodshead and others denouncing McCovern as responsible for losing the war in Indochina and therefore culpable in Cambodia. Old friends from the antiwar drive and the 1972 McGovern presidential campaign called to seek clarification of what he had in mind.

The Wall Street journal, in an editorial, "McCovern the Hawk," denounced his statements as "truly mindboggling." The Boston Globe, on the other hand, gave him credit editorially for focusing attention on "maybe the most heinous script being played out on the world stage."

At a luncheon given by the Foreign Relations Committee Wednesday, a group of visiting journalists from China denounced McGovern to his face, charging that the situation in Cambodia is purely an "internal matter." China is supplying military ais to Cambodia's war with Vietnam, which is currently allied with China's archrival, the Soviet Union. Severl senators argued in reply that China should be concerned, and use its influence to stop the Cambodia carnage.

Estimates of the number of deaths in Cambodia by executive, starvation and other horrors vary widely. The country is sealed to the world. The State Department, which has submitted a 1 1/2-inch-thick report to the United Nations on human rights violations there, said at least "scores, probably hundreds of thousands" have been killed and that the internal bloodletting continues.

McGovern said yesterday he has "no illusions" about creation of a UN military force to intervene in Cambodia, since China would veto such a plan even if others support it. He said he hopes for a "total embargo" on all shipments to Cambodia and an international appeal to China to use its influence until the killing stops.

"I realize I hit a sensitive nerve," McCovern said of the reation to his statements this week. "I am reassured in the knowledge that a lot of people care."