Sen. George McGovern (D-S.D.) tangled with the State Department once again yesterday over military intervention in Indochina. But in a startling reversal of their roles from the 1960s, this time McGovern advocated intervention and the State Department argued against it.

The topic was Cambodia rather than Vietnam, and McGovern made clear that he wasn't proposing that the United States send in the Marines. Nonetheless, he said, the reported "genocide" of Cambodians at the hands of their government justifies consideration of an international military force to "knock this regime out of power."

Deputy Assistant Secretary of State Robert B. Oakley, testifying for the Carter administration, quickly told McGovern that the option of military intervention is not being considered anywhere, except possibly in Hanoi, whose armed forces have engaged in recent waves of warfare with Cambodia.

The United States' ability to influence events in Indochina is very limited, Oakley said at another point. From the experience of the 1960s "we've learned a lot about the level of appropriate U.S. involvement" and now the government is being very cautious.

Douglas Pike, a Foreign Service officer who was a prominent government analyst of Indochinese communism during the 1960s, cautioned McGovern that a "quick, surgical takeout" of the Cambodian regime is probably impossible. Pike said the Vietnamese tried a "quick judo chop" against the Cambodian regime with 60,000 troops a year ago, but this "failed abysmally."

The government of Cambodia, which now calls itself Democratic Kampuchea, consists of nine people at the top, no regional organization that is discernible, and a communal structure "in the style of the 14th century" in villages throughout the land, Pike said. An invading force would have to take control of every village, he added, and such an enterprise of uncertain prospects would be "stepping deeper into the swamp."

In the exchange of views before a Senate Foreign Relations subcommittee, McGovern said there is "a vast difference" between the current situation in Cambodia, where extreme oppression is taking place, and the situation in which the United States intervened in Vietnam. He said military intervention is justified only in the most extreme circumstances but that Cambodia "is the most extreme I've ever heard of . . . Based on the percentage of the population that appears to have died, this makes Hitler's operations look tame."

Oakley declined to give an official estimate of the death toll in Cambodia because of a lack of precise information and the likehood that an "official" figure would become a source of controversy and debate. "The U.S. government is confident that scores, probably hundreds of thousands of people, have been killed" since the communist takeover in 1975, he said.

McGovern, according to aides, has been deeply concerned for many months about the Cambodian situation, believing that it is in part a legacy of the Vietnam war. Earlier this year he sponsored an amendment to the State Department authorization bill calling for unspecified "multilateral action" by the United Nations and the bilateral action by those nations with influence to end "brutal and inhumane practices" in that Asian nations. Aides said yesterday's hearing was the first time he suggested military intervention.

Oakley said U.S. intelligence agencies report that "scores of thousands" of troops on each side are engaged in the current battles between Vietnam and Cambodia, with aircraft, artillery and other modern weapons being used, especially on the Vietnamese side. Calling it "a major conflict," he sad that Cambodia continues to fight very fiercely.