The Veterans of Foreign Wars broke ranks with other major veterans groups yesterday by stepping into the Agent Orange controversy and throwing its weight behind claims that the chemical defoliant caused serious health hazards to Vietnam servicemen.

James Currieo, VFW commander in chief, said the decision to endorse legislation that would compensate alleged victims of the herbicide came after a membership resolution and his own conclusion that the Veterans Administration was "foot dragging" on the controversial issue.

The government has steadfastly denied that Agent Orange was responsible for the disabilities, ranging from cancer and nervous disorders to malformed children.

Outgoing VA administrator Robert P. Nimmo charged that Vietnam veterans were seeking "preferential coddling" not available to veterans of earlier wars. He also contended that scientific evidence indicated that Agent Orange caused no medical adversities worse than an ailment similar to "teen-age acne."

Coincidentally, President Reagan yesterday nominated Assistant Army Secretary Harry N. Walters, a former West Point football player, to replace Nimmo. Story on Page A4.

The VFW is one of the nation's two largest and most powerful veterans organizations. The other, the American Legion, contends that there is no "conclusive scientific evidence" linking Agent Orange to veterans' ailments.

About 15,500 Vietnam veterans have claimed Agent Orange disabilities.

Between 1961 and 1971 the United States sprayed about 12 million gallons of defoliant on Vietnam in an effort to destroy enemy crops and deny the Vietcong the protective cover of jungle foliage. The spraying covered an area equal to the size of Massachusetts.

Ironically, the spraying was stopped in 1971 after a rash of South Vietnamese newspaper reports that women in the area were giving birth to deformed babies.

Rep. Thomas A. Daschle (D-S.D.), a Vietnam veteran and sponsor of a bill that would place the burden of proof on the VA in Agent Orange cases, said the VFW's endorsement would give his bill "powerful new credibility" in Congress.

Daschle called the government's failure to deal with the issue "unconscionable."

He said the VA's intransigence had put Vietnam veterans in the "bizarre position" of becoming the new protesters of the '80s, replacing the anti-war demonstrators of the '60s and receiving the "same sort of government disdain."

"We are not setting a very good example to future generations by denying benefits to those who went and fought," Daschle said.

The Agent Orange veterans organized in the late '70s after they began developing ailments at unusually young ages. Like their "hell-no-we-won't-go" brothers of the '60s, they have organized sit-ins, marches on Washington and other demonstrations.

To the government, however, the Agent Orange controversy has been treated like Pandora's box -- one that, if opened, would lead endlessly into extremely expensive claims not only for Vietnam veterans but also for the "atomic veterans" who contend that they are suffering from radiation exposure in early nuclear tests.

At one point, Nimmo said the claims on Agent Orange alone could cost hundreds of millions of dollars and stretch into the next century.

Daschle denies the claims would be nearly that large, but could not place a figure on the cost. "The real nub of this," Daschle said, "is that if someone walks into a VA hospital with symptoms of this ailment, isn't it simply right to give him attention regardless of where he got them instead of miring down in a bureaucratic mess?"

Government officials also are nervous that an acknowledgment that American servicemen suffered from the defoliant could cause international reaction that the United States conducted chemical warfare in Vietnam.

The first defoliants were introduced to the war in top secrecy under the code name, Ranch Hand. The decision caused a major debate among Kennedy administration officials who were concerned about the chemical warfare implications.

There are no indications that the government knew the defoliant was harmful to humans. But former secretary of state Dean Rusk argued privately that use of the defoliant to destroy crops was contrary to the goal of winning the support of the people in a guerrilla war. He was overruled.