Eric Sevareid has come to represent a one-man coalition of enlightened common sense. What we learned to expect of him from his commentator's pulpit was not so much the taking of a position as the taking of The Great Position - an ultimate, melting-pot statement of opinion produced from the remnants of all available viewpoints.

Maybe 10 minutes later you couldn't remember just what the position had been; but while you were hearing it, it seemed incredibly reasonable and considered and, somehow, national rather than personal - a touchstone for the mainstream.

When that venerable prune face showed up on the screen, you knew you weren't in for a cheap thrill or bit of fey japery. It was, instead, a little like being confronted with your old civics professor, pop-quiz in hand, at that. "Eric went from facts to intelligence to wisdom," reported Charles Kuralt will say of Sevareid on a CBS News special to be seen December 13. But he also went from wit to sage to eminence; he became the very avatar of sober significance, yet maintained the requisite oatmeal immediacy we require of television performers.

And so a number of interlocking eras may be ending as Sevareid's last commentary for the CBS Evening News airs tonight. With Sevareid's forced retirement (at 65) from an active role in CBS News, we lose the sense of stability that his recurring presence conveyed. It's a tacit victory for the forces of fractionalism and specialization in broadcasting and American life generally.

Certainly the momentousness of the occasion is not going unnoticed. In a very rare gesture, and one made purely out of respect, NBC News got permission from CBS to show about 40 seconds from Sevareid's farewell on the NBC Nightly News tonight (in some cities, including Washington, the two network's newscasts air at the same time).

Nightly News producer Joseph Angotti says of the last Sevareid piece, "It's just absolutely beautiful."

At ABC, Av Westin. producer of the ABC Evening News, said he "hadn't really considered" yesterday whether or not to take note Sevareid's retirement on his show. Howard K. Smith, who becomes the last remaining nightly network commentator, is on vacation, so Westin expects that either Harry Reasoner or Barbara Walters will "probably say something."

Sanford Sokolow, CBS News vice president in Washington, says that Sevareid taped his last commentary on Sunday, partly to avoid "all the sobbing and handshakes" of a twilight's last gleaming.

Is it possible there will be so much hard news that the Sevareid farewell will have to postponed? "World War III would have to break out to bump that piece off the air," Sokolow says.

It will be "personal" note, and CBS has given Sevareid about 45 seconds beyond his usual two and a quarter minutes to deliver it. All Sevareid will let us quote in advance is his sign-off, after 38 years with CBS and 14 years on the Cronkite newscast:

"This is Eric Sevareid. Thank you, and good-bye."

Sevareid retires even as the Geraldo Riveras of TV journalism ascend - at a time when network TV news is more noisily competitive and show-bizzy than ever. Sevareid was an obstacle to the cult of personality, partly by virtue of appearing not to have any; instead, he came across as an idealized granite composite of nine Supreme Court Justices. He wasn't one to mess around, wear gaudy shirts or hit us over the head with his own name.

And yet Sevareid himself, one of the few remaining graduates of the revered Edward R. Murrow school of broadcast journalism, isn't despairing about the present or future of TV news, even though its trivializing tendencies would seem anathema to him.

"It is a problem of course," Sevareid said yesterday. "Television is the most personal journalism there has ever been, more personal even than radio, and you cannot get away from that personality aspect of it. As competition gets more severe, I think there'll be an even higher premium on personality in TV news. That doesn't have to mean that everybody on the air will be a charming idiot. There are plenty of able reporters and writers who are also good on television.

"Within three of four years, all the news people on television will be people like that - somebody who projects a personality but who is believable as well as attractive."

In a series of valedictory CBS Radio interviews with Walter Cronkite, Sevareid decried the "glut of happiness" in some TV newscasts but still thinks the happy news craze has cooled.

"I don't think it's getting worse, not even on local stations," Sevareid says. "There's bound to be some of it, because in a long period of peace and prosperity, it's inevitable that people turn inward. You can't keep dealing with the great cosmic questions because people weary of them. So you have new magazines devoted to personalities and newspapers like The Washington Post giving more space to gossip.

"When something really serious happens, some great world calamity, then you'll see a change. I don't wish for such things to happen, but they do."

With Sevareid stepping down the demise of the broadcast commentator, and perhaps of the gentleman pournalist as well, seems at hand. Commentary is more and more something that creeps into regular news reports. Sevareid thinks commentary's golden era was before television, anyway.

"Commentary flowered in radio days, partly because radio time was cheaper. You had all kinds of people doing 15-minute commentaries. It didn't translate into TV very well. In radio I had four minutes or so, and I wasn't buried in a hard-news program. I come along now as a little postscript, and I've had to run with the news of the day as close as I could.

"And the time they give me is not really a natural length," Sevareid says. "As a result, you're bound to sound terribly positive and almost smug. That's part of the price of compression; it gives you no chance to loosen up a bit. And of course I look like the great stone face anyway."

Sevareid is by no means going to vanish from the public eye. He has already taped narration for a public TV documentary on the human brain and will soon start work on a 16-part, Mobil-sponsored syndicated TV series called "Between the Wars," which will be telecast beginning in April.

"I won't leave CBS entirely," Sevareid also notes. "I'm just moving my office one flight up, to a littler one. I'll spend a couple days a week kibitzing as a CBS News consultant, annoying people here and there with memos."

In bidding its farewell, Newsweek dubbed Sevareid "without doubt the most imposing of all broadcast commentators." But New York magazine sent a sniper with its bon boyage: "Why I Won't Miss Eric Sevareid." Sevareid hasn't read that one but says, "It's almost a relief to know someone's giving me hell; everyone's been so nice it was beginning to make me nervous."

It's easy to mistake for blandness Sevareid's even-headed and unifying soundness; his opinions were indeed homogenized, at least as opposed to being rancorous, but that helps prove how sensitive he remained to the breadth of his constituency. Those who think they won't give him another thought after tonight might be surprised to find themselves giving him several in the rocky years to come. Eric Sevareid could almost have made his parting words those of another respected sage: "May the force be with you."

He is a knight of an old order.