A piece on Washington-based activist Mitch Snyder was in the works at "60 Minutes" long before Snyder began a hunger strike in behalf of the homeless and impoverished in this country, but the report has taken on a grim timeliness. It will air tomorrow night (at 7 on Channel 9) on the 51st day of that strike, and there is now such urgency about Snyder's condition that Mike Wallace will be standing by Sunday to do an updated live conclusion to the piece if one is necessary.

Produced by the gifted and dedicated filmmaking team of Paul and Holly Fine, the profile of Snyder was filmed last spring as Snyder went about what he has taken on as his business: caring for, defending and trying to elicit public concern over America's homeless, whose ranks include the utterly impoverished, de-institutionalized mental patients unable to fend for themselves, and destitute outcasts.

Yesterday, Wallace phoned Snyder from CBS News in New York and spoke with him for about 10 minutes, gently imploring him to discontinue the hunger strike Snyder began to protest what he says is federal reneging on a promise of adequate facilities. Wallace said yesterday after making the call, "I like him. He's a fascinating man. I think he believes deeply in what he's doing."

Wallace said Snyder indicated "he thinks he's making progress" with the Department of Defense, which Snyder has accused of taking money that was promised for shelters and using it instead for routine department maintenance.

It may be that in the 20th century, the only way to make an impact as a humanitarian is also to be a fanatic. Snyder certainly seems that in the "60 Minutes" profile, and a media-wise fanatic as well, but also a man of heroic resolve. He doesn't romanticize the disadvantaged. "There's people in this building who would kill you," he tells Wallace as the two of them tour Snyder's makeshift shelter. All he is trying to prevent, Snyder says, is "needless human suffering."

Under the compassionate eye of the Fines' camera, Snyder helps a man whose foot is hideously frostbitten, joins in a forage through garbage to find food for what has become his flock, and tries to calm a man experiencing a violent seizure. The Fines are able to watch these things without flinching and also without making the viewer feel like a morbid voyeur. Not that you don't squirm, and recoil, and despair.

Wallace, whose reputation is that of the dogged, trench-coated investigator, proves strikingly right for this assignment. He walks with Snyder through a protest march (one banner says simply, "Shelter") and interviews him as only a master conversationalist could. Too seldom, it could be argued, does "60 Minutes" air pieces that really have the capacity to cause the middle-class audience productive discomfort. This one does.

It's TV news, not the White House, that we must rely on for appeals to conscience. The official battle cry of the '80s has little to do with being one's brother's keeper. This "60 Minutes" profile of Snyder is no puffy propaganda piece; it's good journalism. What it lets you do is look directly into the eyes of hopelessness and then, directly into the eyes of hope.