Defense Secretary Casper W. Weinberger declared yesterday that he favors deploying neutron warheads in Europe partly because they "could do quite a lot" offset the Soviet advantage in tanks along the NATO front.

Weinberger, in an interview with The Washington Post, thus underscored his personal commitment to the weapon despite the controversy that erupted in Europe last week when he first set forth his position. The State Department felt compelled to cable North Atlantic Treaty Organization capitals to reassure them that Weinberger's remarks did not represent any change in the government position on the weapon.

The new defense secretary stressed that he was still speaking for himself, not President Reagan, since no formal administration decision has yet been made. He added that European allies would be consulted before any deployment of neutron weapons took place.

Nevertheless, he repeated that he felt former president Carter made a mistake by not going ahead with the production and deployment of enhanced radiation warheads, especially since NATO nations seemed ready to accept them, though in some cases reluctantly. The weapon has been held in the development stage, although it could be put into production quickly.

Enhanced radiation weapons are designed to kill enemy troops without devastating the countryside in the process. Critics contend this would make it overly tempting for military commanders to fire the weapons on European soil, thus increasing the risk of a nuclear war; that is one reason some European countries have been nervous about their deployment. Backers of neutron warheads counter that they would deter the Soviets from attacking Europe in the first place.

"The American government made a policy determination some time ago that it was good and necessary and a helpful addition to the strength of theater nuclear forces," Weinberger said of the neutron warhead.

"When you look at the number of Russian tanks, and the other items," he continued in talking of the East-West balance along the NATO front, "the enhanced radiation warhead could do quite a lot to restore some kind of balance there. And I believe that's one of the reasons the Russians are reacting so strongly to this slight suggestion."

Weinberger said he faults the Carter administration for its "sudden withdrawal" in 1978 of its deployment plan. The "reversal," Weinberger contended, "understandably confused and angered the NATO allies, particularly Germany."

Asked if he favored deploying neutron warheads even if the United States also deploys nuclear-tipped cruise missiles on the ground in Europe, Weinberger answered in the affirmative. "The enhanced radiation weapon adds a great deal to the strength from a number of points of view, one of which is deterrence."

If deterrence should fail and Soviet tanks rumbled across the NATO line, the defense secretary contended, neutron radiation would pierce the armor of the enemy tanks and kill their crews without at the same time contaminating the surrounding ground to the point that friendly forces could not use it.

"I think it's a very good addition," Weinberger said, but, in referring to NATO allies, added: "It's nothing that we are going to force on them. It's nothing that we are going to say, 'All right. It's here. You've got to take it or leave it.' It's got to be after a process of consultation and agreement. Some areas seem to be against it; some areas seem to be strongly for."

The defense secretary gave his views on the neutron warheads and several other subjects while sitting at a round table beside the huge desk of World War I Army General "Black Jack" Pershing, which goes with the office on the third floor of the Pentagon. The 63-year-old Weinberger said he has been working from 7 in the morning until 7 or 8 at night, mostly on the fiscal 1981 and 1982 defense budgets inherited from the Carter Administration.

"My wife thinks that's just being badly organized, that any well-organized person ought to be able to leave at 5. And I guess that's right. But there is a lot to get all at once."

Harold Brown, Weinberger's predecessor, used to start his equally long day with a swim in the Pentagon pool. Weinberger said he does not do that, but "I do a little running each morning before I come to work. Get up around 5:30 or so."

He declined to be specific about his primary preoccupation -- deciding how much should be added to the Carter defense budgets.Other defense officials have said there are proposals in Weinberger's in-basket to add $6.4 billion to the fiscal 1981 defense budget and about $25 billion to fiscal 1982, raising those requests to $177.6 billion and $221.4 billion respectively.

Weinberger hinted he is about to live up to his budget-cutting reputation as "Cap the Knife" when it comes to some of the military's requests for add-ons, declaring: "We had a Cabinet meeting all morning about what size cuts can be made. Obviously defense is not immune from that process."

While there definitely will be substantial net additions to the Carter defense budgets, Weinberger said he is trying to hold them down though such economies as contracting more military work out to civilian firms that could do it cheaper, reducing the number of consultants on the Pentagon payroll, and "some base realignments."

You mean closing military bases? Weinberger was asked.

"Watch your language!" he exclaimed with a smile. "Those always involve heavy political pressures. I've got some delegations already lined up to call on me before the subject has ever been mentioned publicly."