THERE HAS been plenty of shouting and jeering -- some thrown eggs, some thrown punches, some thrown-out hecklers -- this week; Demonstration America is back in the news. One of the exchanges, Ronald Reagan's in the South Bronx, seems to us especially worth noting in that it had a constructive side. Surely the people in the South Bronx who turned up to protest the way in which they and their desolate terrain have become every politician's national photo-opportunity have a point -- a large one. Their poverty and burnt-out surroundings have served as backdrop for too long; and this is the ultimate and finally most degrading kind of exploitation: "Say, would you folks mind if we just got some shots of you, a little footage with the candidate here? That's it . . . look this way . . . no, don't smile please."

Well, there's not all that much to smile about in the part of New York where Mr. Reagan took his tour the other day. And it is good that those people made known their disgust with the trend to use the place as photographic background, as you might pose the family in front of a cardboard replica of the Alps, and then move on. But we thought Mr. Reagan's shouting back wasn't so bad either. What he was shouting, after all, was not abuse, but an angry demand that he be given a chance to take the case.

You can argue that, from the beginning, the South Bronx, with its uniquely dismal prospects for redemption, was the wrong place to make a touchstone of the federal government's capacity to do something about urban poverty.There were poverty-ravaged places where more progress (and more hope) could have been generated in a shorter time. But Ronald Reagan didn't make that original choice of symbolic betting. Jimmy Carter did. And, for all the justice of the local inhabitants' complaints about being put on the political grand tour, there is a sense in which it is good that Mr. Reagan went there. To some extent anyway, it represents a right direction in his campaign, just as his entreaty-speech to the National Urban League that same day did.

It is possible to argue with the gospel according to Kemp-Roth and still admire the fact that Mr. Reagan took his message and his arguments to urban black America that day, that he made his earnest pitch and his pledges. He said that blacks are getting a miserable economic and social deal from the Democrats, that the conditions they deplore are testimony to that and that his government would drastically alter their condition, would make their lives materially better. You can believe it or not. But no one can fail to note the straightforwardness of the approach. No subliminal campaigning here. No euphemistically named "Southern Strategy." No big white Country Club members-only wink. No code.

Let up be plain: this would be as good a year as any for one of those two-tiered, sly race-baiting campaigns: black anger and outbursts in several cities, anxiety and competition for blue-collar jobs -- the elements for it are there. And so is the rationale, if he wanted to invoke it, for Ronald Reagan to "write-off" the black vote. Evidently he has decided not to do so, but rather to challenge the Democrats for that vote and to pitch for it on forthright economic-interest terms. In that sense, anyway, the South Bronx confrontation was good news.