IT IS ESTIMATED that before the senators vote on the labor-law bill next month, more than 3 million letters and postcards about it will have been delivered to their offices. If you take the estimates at face value, that is a tremendous outpouring of public interest in one piece of legislation. You might even think that this controversial bill is a major topic of breadfast-table conversation across the nation. After all, how often do 3 million people get sufficiently stirred up about anything to write their senators?

The truth, of course, is that hundreds of thousands of these communications will be identical, except for the names and addresses of those who sent them. The mail has been generated on one side by business organizations that oppose the bill and on the other by labor unions that favor it. Form letters and even preprinted, stamped postcards have been handed out in offices, plants and meetings across the country so those who care to express their sentiments, or dare not refuse to sign, can get counted in Washington.

Getting counted is precisely what is happening, too. In some offices on Capitol Hill, some poor souls hand count the daily flood of mail. In others, estimates based on how many boxes or bags came are acceptable to the boss. This would seem to be an enormous waste of paper, money and time. Yet lobbyists on both sides of this particular issue believe that some members of Congress are heavily influenced by how much mail they get. Labor leaders think the mail generated by business groups explains the defeat in recent years of some bills labor supported. So they have geared up to match the corporations' sophisticated computer operations. Thus, about 1.5 million postcards signed by labor-union members are already delivered or ready to go.

We would like to think that the lobbyists are wrong in their belief. It is at war with the old lesson of civics courses that one carefully drafted, personal letter to an elected official is worth 50, or even 100, form letters. But we wonder. The success of mass mailings by political fund-raisers and certain commercial ventures suggests that average citizens are swayed by form letters. Why should politicians be different? If one of them gets 30,000 postcards on one side of an issue, what is he or she to think? They came - or, at least, they appeared to come - from 30,000 potential voters. If senators are influenced by such an outpouring, is the current blizzard of mail on an issue that is of little personal concern to most Americans only the beginning? How many million postcards could be generated by a sophisticated effort in behalf of a subject such as abortion or the Social Security tax? With the post office andcongressional aides in mind, we shudder at the thought.