AMONG THE LETTERS to the Editor today are several taking issue with our support of the American Civil Liberties Union for its decision to defend free speech for the American Nazi Party and the Ku Klux Klan. The theme of the letters is that those two groups, especially the Nazis, have no legal or moral right to hold demonstrations or to promote their racist views. A secondary theme is that no respectable organization would support their claim to such a right. These themes conflict so profoundly with our own conception of what freedom of speech entails that we wish to comment on them.

We start by conceding that it is by no means clear that the ACLU will be successful in defending the constitutional rights of the Nazis to march in full regalia through Skokie, Ill., or the right of the Klan to hold a rally on public property adjoining a desegregated school in Mississippi. The Supreme Court, unfortunately, has followed a zigzag path in applying the First Amendment's guarantee of freedom of speech in situations in which groups want to express views that so deeply offend other citizens. The ACLU, however, is being consistent in applying to the Nazis and the Klan the same - and, in our view, correct - reading of the First Amendment that it has applied in the past to groups of a wolly different political character.

That reading is not a comfortable one to live with. It requires citizens, such as those who have written to us, to tolerate expressions they find terribly offensive. It grows out of the proposition that Justice Oliver Wendell Holmes stated so well 50 years ago: ". . . if there is any principle of the Constitution that more imperatively calls for attachment than any other it is the principle of free thought - not free thought for those who agree with us but freedom for the thought that we hate,"

The rationale for this is that in a free society public policy will develop best through the open clash of ideas, evil ideas as well as benign ones. Noxious doctrines die more quickly in the sunlight than in the shade, and public exposure of the ideas of the Nazis and the Klan make them less, rather than more, dangerous.

There are limits on free speech, as our letter writers note. But those, in our view, rarely relate to its political content. There is a difference between verbally abusing an individual by name and abusing a group to which he belongs. There is a difference between arguing for a particular creeds, as do the Nazis, and attempting to put it into operation by force. The First Amendment, as we read it, permits government to suppress speech only when there is substantial reason to believe that it will cause, almost immediately, a serious and palpable public danger or evil event.

Far more dangerous is the argument that the Nazi Party has forfeited First Amendment protection by advocating so vile a policy. That argument opens the way to the suppression of all kinds of views on the grounds that these views, too, are deeply offensive to some part of society. We have no doubt, for example, that many Americans believe that those supporting legal abortion are really advocating mass murder. And we know that many of them find speeches and articles about the availability of abortion services inhumane and deeply repugnant. From their point of view, could not a case be made on the basis of some of our letter writers' logic denying pro-abortion people First Amendment protection?

The point is not the self-evident one that the Nazi Party and the Klan are founts of opinions that are alien to the principles by which this nation lives. The point is that their right to express their terrible opinions rests on one of those very principles: the right of free speech. To deny it to them is to weaken it for the rest of us. And that is why the ACLU is doing the right thing.