Sixteen million Americans get 22 minutes worth of news weekday evenings from Tom Brokaw, the co-anchor of "NBC Nightly News." His employers pay him well--an estimated base of $1.2 million per year. That puts Brokaw at least five times above the salary of President Reagan. It should also put him above the overt taking of sides in political firefights against Reagan and his policies.

Brokaw wants it both ways: a high-visibility reporting job and high-decibel blasts against those he is covering. Is he a reporter or commentator?

In a Q&A format in the April issue of Mother Jones magazine, Brokaw says that Reagan's values are "pretty simplistic. Pretty old-fashioned. And I don't think they have much application to what's currently wrong or troubling a lot of people. His values are kind of Norman Rockwell- ish, Reader's Digest America, 1924. I'm not exaggerating that: thrift, hard work, a lot less government, kindness toward your neighbors, and it'll all work out somehow."

Brokaw the reporter becomes Brokaw the opinionator on other subjects. He finds it "quite outrageous" that the business community braced itself for the "disastrous consequences" of supply-side economics "but did not publicly warn the rest of the country about (it). They knew it simply could not work. But what they did was look to their own little life raft and not to anyone else's."

On El Salvador, the Reagan administration record rates another "outrageous." On abortion, Brokaw says "it comes down to whether a woman has a right to control her own body."

I don't know the Nielsen reasons why some viewers prefer Brokaw at NBC to Dan Rather at CBS or Frank Reynolds at ABC. But I know that Brokaw has provided a justification for a fast turning to the other programs.

Anchor jobs are reportorial. Airtight objectivity is impossible for any newsperson, but Brokaw's outburst is a scattershot venting that can't help eating into the public trust of the media. The suspicion is now raised that whenever Brokaw reports a critical story about the Reagan administration he is on a personal mission.

And if Brokaw is getting away with it, who can guess how many others are?

The public is sophisticated about the news business, knowing the differences between reporting and commentary. It understands--and demands--the separation.

Brokaw let himself be baited by the Mother Jones interviewer. "Beyond your role as anchorperson," he was asked in one question, "what's your gut response?" In another, he was asked for his "visceral reaction." He should have summoned the discipline to say "no comment."

That Brokaw was willing to speak from the gut appears to have given Mother Jones the idea that it had a scoop. Its press release for the interview hypes that "Tom Brokaw steps away from the role of the scrupulously objective reporter with surprisingly candid comments on Ronald Reagan, El Salvador. . . ." The interview describes Brokaw as one who "holds to the conviction that his brains and decency are enough to transform television news from banality to brilliance."

While awaiting the transformation, I recall an evening a few years ago when I was seated at dinner with Frank Reynolds. Now I'll get to know his politics, I thought to myself. Is he a moderate, lefty, right- winger or a stew?

I never found out. Even in relaxed company, in off-the-record conversation, Reynolds wasn't trading in opinions about the events he was paid to cover as a reporter. If he were delivering the commentary segment of the news, which only ABC of the three networks doesn't have, it might have been different. Or if he were in print journalism writing editorials or columns. Then he might have held forth, though Reynolds, being a polite citizen, would likely have spared his dinner companions any attempts at off-duty genius.

No one expects that Tom Brokaw self- impose a gag rule. Much of his interview with Mother Jones is of passing interest, if your taste is for anecdotes from a man who is, in the magazine's words, a "friend of celebrities and statesmen." After that, though, it's only a reporter running off at the mouth. In this kind of running, calories don't get burned. The public does.