YOU MAY NOT have been aware of it, but "an invasion of southern states" is under way, and the invaders are none other than "liberal northern political organizers." That, at least, is the news we received in a letter the other day from one Fitzhugh Lee in behalf of Southerners for Reagan. "One hundred years ago," Mr. Lee says, "they would have been known as 'carpetbaggers.' Now they call themselves 'civil rights' workers." They have "come south to destroy our heritage and way of life."

All of this has the tone of something that got stuck in the mail 20-odd years ago. Mr. Lee descries his political opponents as "outside agitators" -- the favored term for supporters of civil rights in the old days when some white southerners used almost any means to prevent peaceful integration. He talks of "out-of-state northern liberals who have come south to destroy our heritage and way of life." All these are code words, innocent enough in isolation but, when put into the context of the southern history Mr. Lee likes to invoke, ugly in their implications.

What, after all, is wrong with encouraging qualified black voters to register to vote in the state of their residence? What is wrong with raising money in one state to spend in another (as Mr. Lee himself is doing)? And what is this "heritage" Mr. Lee is trying to protect? A heritage in which blacks are not part of the political process?

Of course there's nothing wrong for a Republican, when he sees the Democrats register a lot of their voters, to want to register of lot of his own. That's true even if the Democrats are of one race and the Republicans another. "I know," says Mr. Lee in his letter, "you understand what I'm talking about." Mr. Lee says that his organization has "total independence from the national campaign." But it might be in the interest of the Reagan-Bush campaign to make it clear that they don't have anything to do with these code-word appeals to segregationist sentiment.