The case for a second arms-control treaty with Russia finds powerful support in the accident that downed a nuclear-powered Soviet satellite over Canada last week. By emphasizing the importance of accidents - the so-called bungle factor - the episode works to discredit those opponents of arms control who demand an absolutely 100 percent perfect treaty. In addition, the episode underlines the importance of continuing Soviet-American cooperation in monitoring nuclear devices and satellites.

Critics of the arms-control negotiations have centered their fire on the vulnerability of this country's force of roughly 1,000 land-based missiles. In their view, Russia is acquiring enough missiles with enough destuctive power and enough accuracy to wipe out that whole force in a single strike.

Moscow, the theory continues, would then hold this country and its civilian population hostage. The Russians would use the threat of a second strike to extract major diplomatic concessions from Washington. Indeed, according to the theory, even the threat of being able to launch a first strike with impunity would give Russia a decisive edge in world politics.

The glaring weakness in such catastrophic scenarios is that they bury the bungle factor beneath a skyscraper of assumptions. It is assumed that the Soviet Union has its weapons so perfectly placed and targeted that there would be almost no misses. It is assumed that the United States is taken by surprise and does not, on first warning of the attack, retaliate with its full array of missiles, including the land-based force.

It is further assumed that after the land-based force is wiped out, American policymakers would do nothing. It is assumed American leaders would not use the devastating force of 160 missiles in each of the 25 Poseidon submarines likely, under conservative estimates, to survive the first attack. It is also assumed that American leaders would not use the large bomber fleet equipped with nuclear weapons that have destructive power equal to about half of Russia's total missile force. Most important of all, it is assumed that Russian leaders would make all these assumptions - would run the truly horrendous risk entailed in a first strike with weapons that had (by definition) never been used before.

The accident of Canada injects a little realism into that preposterous scenario. The mishap to a well-tested satellite system long in use shows that there is likely to be a bungle at every step in the chain of perfect assumptions. It reminds us that Russian leaders - familiar with countless such difficulties - are apt to be far more wary of bungles than are Americans. It shows us that the notion of a Soviet strike comes out of the realm of science fiction.

So to be useful the next arms-control accord does not have to afford absolute protection to this country's land-based missile force forever. It is enough that it curtails the projects for bigger and better weapons now on the boards, and begins the scaling-down of the grossly excessive armories held by both super-powers.

The more so, because of the value of cooperation. The United States and Russia were jointly aware of the falling observer satellite for almost a month before it finally came down in Canada. The exchange of information made a potentially alarming event a good deal less worrisome.

Far more detailed cooperation than that is part and parcel of the arms-control agreement negotiated in 1972, and the one now being completed in the Geneva talks. Those agreements stipulate levels of weapons for both Russia and the United States.

They provide for monitoring by each country, and forbid the other to interfere with the monitoring. They establish a kind of court - a joint commission - whereby each side can take complaints and demand explanations from the other. Arms-control agreement, in short, institutionalizes Soviet-American cooperation in the area of monitoring strategic weapons.

What all this says to me is that an arms-control treaty does not have to afford total security to pass muster. Significant gains are made if only the present monitoring system is maintained, and the way is opened to cut off projected weapons developments. Failure to reach an accord would yield an unconstrained arms race and the end of joint monitoring. In other words, any agreement apt to come from the present Geneva talks is far better than no agreement.