President Reagan's proposal to focus U.S. scientific skill on ways to shoot down Soviet missiles represents a bold gamble that could lead to a revolutionary military breakthrough or make his already controversial defense policies even more so.

In announcing his plan last night for an all-out research program to see if "we could intercept and destroy strategic ballistic missiles before they reached our own soil or that of our allies," Reagan sought to raise the notion that the wave of the future could be a shift from offensive to defensive weapons development.

Such an idea could have some popular appeal. It could take some attention away from weapons of mass destruction, such as the new MX missile. It could also take some steam out of the nuclear freeze movement. It might make people feel more secure, the president noted last night, in that it offers an alternative to automatic and instant retaliation if Soviet missiles are fired.

But Reagan's proposal also could reopen the bitter debate that flourished here in 1969 and 1970 over whether this country should try to build an anti-ballistic missile (ABM) defense system.

In 1972 the United States and the Soviet Union finally signed a treaty allowing each country to build a defense around a single city or military base, and banning anything more. The United States did not even activate the one site allowed because it was widely assumed then that ABMs don't work and that the offense can always overwhelm the defense.

The idea behind the ABM treaty was that defense was potentially dangerous and destabilizing because it might lead either superpower to think it could safely attack, then shoot down the other side's remaining missiles when it tried to retaliate. In short, the United States and Soviet Union agreed to leave their countries hostage so as to ensure that neither would strike first.

In his speech last night Reagan acknowledged all the pitfalls. It is still not at all clear that missiles can be shot down, and it may take until the end of the century to figure out if it is possible. And, he said, "I clearly recognize that . . . if paired with offensive systems, they ABMs can be viewed as fostering an aggressive policy, and no one wants that."

Nevertheless, it is precisely those issues on which critics undoubtedly will focus: whether it will lead in the end to a breach of the ABM treaty and a potentially destabilizing quest by both superpowers for nuclear superiority based on having a defense as well as an offense.

Such an accelerated program is certain to be even more expensive than the $1 billion already spent annually on such research. There will be charges that countermeasures can always be developed against any defense, and that the program is so long-range that another administration will probably stop it before it can produce much.

On the other hand, Reagan has done something rare. He has launched a new technological crusade, not as specific as the race to the moon, but at least potentially important, to see if American technological prowess can achieve a radical shift in emphasis that might "free the world from the threat of nuclear war."

Because this project was launched from the White House, it is apt to be taken more seriously and to be more controversial than if it came from the Pentagon.

Such a crusade is almost certain to rattle the Kremlin because it tends to emphasize American technological strength. Although the Soviets have always seemed more interested than the United States in ABM systems and have made a more vigorous research and development effort, most technical experts believe that the Soviets do not have an appreciable lead on this country.

Many technical specialists believe that if there were ever an ABM race the United States would win. The questions remain, however, of whether any system will really work and whether a country might miscalculate and launch an atomic attack because it thinks, perhaps mistakenly, that its system will work.

Henry Kendall, a physics professor at the Massachusetts Institute of Technology and chairman of the Union of Concerned Scientists, said last night that "the Soviet Union would not stand idly by while we deploy such a system that might effectively disarm them."

Kendall suggested that the Soviets might even try to attack the system before it is completed.

"It is a very provocative system, and a very dangerous nuclear arms race in space would result," he said.