SOME OF THE MOST effective political advertising during the recent campaign was delivered to the public in the mail. New York gubernatorial candidate Lewis Lehrman and Rep. Stanford Parris of Virginia were among those who used this technique especially skillfully. Nationally, Democrats sent out hundreds of thousands of letters labeled "contains important Social Security information" (the information being, of course, charges that Republicans would cut benefits). The Republicans sent out hundreds of thousands of letters from President Reagan to solid Republican voters, urging them to turn out on Election Day.

Campaigning by direct mail, as the consultants call it, is different in one important respect from much other campaigning. It is the MIRV of politics: it allows special messages to be independently targeted. Mr. Lehrman was able to send one message to Jewish voters and another to voters of Italian descent. Mr. Parris sent a letter to voters in southern Fairfax County proclaiming his unalterable opposition to construction of a prison in southern Fairfax County, and he sent a letter to 500 households in Rippon Landing claiming credit for a federal grant to abate odors from a sewage treatment plant there -- that sort of specialized thing.

There has been criticism of this technique. Most political campaigning in recent years has been conducted in television advertising, and whatever bad things you want to say about TV ads -- and they've all been said before -- such ads don't allow specialized appeals to different segments of the electorate. Almost everyone watches TV, and almost everyone will see what you're saying. Campaigning by mail, the critics say, will divide Americans one from another, will exacerbate racial and ethnic divisions, will allow a candidate to say one thing on one side of town and another on the other.

But to see any of this as new is to miss the point. The very archetype of the politician in American folklore is the fellow who talks out of both sides of his mouth. Television introduced a new and, perhaps, temporary universalization of campaigns, forcing candidates to send the same message to everyone at the same time. Now that may be ending, partially because of direct mail and partially because the TV networks may be losing some of their audience to cable, public TV, independent stations and even non-television activities.

Yes, there are dangers to campaigning by mail -- just as there are dangers to every medium of political communication. But elections are adversary contests with vigorous self-regulation. A candidate who crosses the line between fair appeals to a segment of the electorate and unfair appeals to prejudice will probably pay a penalty when his action is brought to public attention. A candidate who says one thing on one side of town and another on the other can be attacked -- often devastatingly -- for that very offense. Voters can distinguish fair from unfair campaign tactics, and the contents of letters sent out in barrages cannot be kept secret from the opposition. We doubt that democracy is going to be ruined through the mailbox any more than it has been through the tube.