President Reagan yesterday contradicted 25 years of government argumentation by contending that a new mobile Soviet missile increases the danger that the Soviets plan to strike first in a future war.

From President John F. Kennedy, inaugurated in 1961, until Reagan's speech in Strasbourg, France, yesterday, the argument from the White House has been that missiles that could survive a first strike and be fired only in retaliation stabilized the balance of terror. The missiles to worry about most, it has been argued through the years, are those that stand still above ground, where they would have to be fired at the first sign of attack or be lost -- "use them or lose them," in the jargon of nuclear strategists. They feared that the United States or the Soviet Union might fire nuclear missiles in response to a false alarm.

Both superpowers have spent billions to try to protect their nuclear forces from a surprise attack. They have taken strategic missiles to sea in submarines, buried them under tons of concrete, and designed them to be mobile and thus hard to locate and hit. The U.S. Air Force considered putting Minuteman intercontinental ballistic missiles on railroad cars in one of its many attempts to achieve mobility, and now is weighing the feasibility of a small "Midgetman" mobile missile.

Reagan said yesterday that the the Soviet Union "has chosen to build nuclear forces clearly designed to strike first and thus disarm their adversary. The Soviet Union is now moving toward deployment of new, mobile, MIRVed missiles which have these capabilities plus the potential to avoid detection, monitoring or arms control verification. In doing this, the Soviet Union is undermining stability and the basis for mutual deterrence."

White House national security affairs adviser Robert C. McFarlane told reporters traveling with Reagan that the president was referring to the SSX24 missile. The Defense Department said in its 1985 "Soviet Military Power" book that the SSX24 "will probably be silo-deployed at first, with initial deployment expected in 1986. Rail mobile deployment could follow by one to two years."

Administration officials said last night that the weapons specialists on the National Security Council staff were taken by surprise when McFarlane and Reagan linked mobility with first-strike intent.

Weapons experts said yesterday that Soviet SSX24s in railroad cars could achieve the accuracy needed to destroy U.S. missiles in a surprise strike. The cars would be halted at prepared spots along the railroad line from which targeting data had been calculated and the gravitational field, which affects the guidance system, analyzed, these specialists said.

However, Spurgeon Keeny Jr., executive director of the nongovernmental Arms Control Association, said "it's a contradiction to describe a mobile system as primarily a first-strike weapon since the point of incurring the cost to achieve mobility is to survive an initial strike by the other side." He added that ways could be negotiated to permit the United States to keep track of the mobile SSX24 as part of a future arms control pact.