The broad consensus that has bound the United States and Soviet Union in a common desire to prevent nuclear war is breaking down, a Soviet expert on nuclear policy says. And, he adds, the deployment of new NATO missiles in Europe increases the danger of accidental nuclear war.
''We used to have one common ground in the past. The United States and the Soviet Union did not want destabilization, did not want to have to take decisions in minutes, did not want to raise the possibility of war by mistake,'' said the official, who spoke on the condition his name not be used.
''Nixon understood this. Ford understood this. Carter understood this. But Reagan chooses to ignore it. And it's a very dangerous thing.''
Specifically, he adds, the introduction of new Pershing II missiles in West Germany - which could hit the Soviet Union within 10 to 12 minutes after firing - adds a dangerous new element to the superpower confrontation.
''This is something new and something extremely dangerous,'' he says, ''when you have to make decisions in minutes. The possibility of mistakes rises.''
His remarks help explain why the Soviet Union continues a full-scale campaign to halt deployment of the new missiles, even as NATO forges ahead with plans to station them. Scheduled to be deployed in West Germany are 108 of the new Pershing II missiles, with the first ones becoming operational this month.
In additional, 464 slower-flying cruise missiles are to be placed in West Germany and other NATO countries. The Soviet Union has broken off negotiations on medium-range nuclear weapons in Europe in protest over the planned deployment.
Western diplomats note that since 1977 the Soviet Union has been able to hit European targets within minutes with SS-20 missiles. They argue that the United States must be able to pose a similar threat to the Soviet Union if its nuclear defense policy is to be credible to its European allies.
Otherwise, they say, the Soviet Union might eventually be able to demand political neutrality from Western Europe - or enforce its will by using the threat of SS-20s.
The Soviets have argued that a rough sort of nuclear parity already exists in Europe - a notion hotly disputed by Western experts.
But there is another major concern, says the Soviet source, that the Soviet Union will have to make a decision ''within minutes'' on whether to retaliate against American nuclear missiles.
The vast stretches of the Atlantic separating the two superpowers formerly guaranteed that each would have adequate time to verify whether a nuclear strike had been launched - and to respond appropriately.
With the response time cut to minutes, he says, ''The urge to push the button in crisis situations will increase.''
Western diplomats say that is a specious argument, because each side has submarine-launched nuclear missiles that give them the power to hit the other's territory with little warning. But the Soviets argue that for one side to be able to hit the other with land-based missiles with little warning upsets the nuclear balance.
''American missiles near our border. That is the most important point,'' the Soviet expert argues.
And, he adds, the USSR will feel compelled to further strengthen its defenses - and place the United States at similar risk.
There is suspicion here, he says, that the US actually wants to force the Soviet Union to make nonproductive investments in nuclear weaponry. But, he says , ''We are forced to take these measures.'' Still he is somewhat hopeful that some accommodation can be found between the two nuclear superpowers.
''Goodwill on both sides'' will prevent a further deterioration of relations, he says.
Does he rule out reaching some accommodation with the Reagan administration?
''I still have hope,'' he concludes.