AMERICAN PROSPERITY seemed to rise steadily, for many years, by a process that people came to consider natural and automatic. But in the late 1960s the improvements began to diminish and, around 1973, the slowdown became severe. Since the beginning of the 1970s, there has been no increase in the average factory worker's take-home pay, the longest plateau of that kind in the country's history. The reasons for it are anything but clear.

The barely averted collapse of the Chrysler Corporation has drawn attention to the troubles in which some large American businesses find themselves. Gov. Jerry Brown of California, in his campaign for the presidency, raises the possibility of a general industrial decline that can be reversed only with close cooperation between the government and private companies. We publish on the opposite page today some of Gov. Brown's views.

Mr. Brown is alluding here to kinds of industrial policies that have been dveloped with notable success in, above all, Japan and France. Both are countries that, unlike the United States, have long traditions of strong central direction of their national economies. Allocating credit, for example, comes easily in a country like France where the government owns most of the big banks. It's harder in this country, where the relationship between government and business tends to be adversary.

But perhaps there's a deeper difference. In a country like France or Japan, people tend to think of economic competition not simply as one company against another, but as their country against a hostile world. That habit of mind creates automatic political support for the central government, allowing it to make and change industrial policy quickly. The question is whether it's necessary, or even desirable, for Americans to talk themselves into that kind of defensive and obedient attitude.

While productivity isn't rising in this country, its general level here is still higher than in France, Japan, or, indeed, any other large country. The alleged decline of American industry is a generalization that won't bear examination of specific cases. Chrysler is certainly in trouble, but General Motors is not. Some of the steel producers are genuinely threatened, but some are as advanced and efficient as any in the world. The distress of the steel industry as a whole is the result of overbuilt capacity and the need to close down aging and obsolescent plants.

One great danger in a national industrial policy is always the terrible temptation to use it to maintain jobs at any cost. If government has the power to prevent a plant from closing, it comes under fierce political pressure to keep doing just that. Here the British experience is instructive.

Gov. Brown's line of argument is not, so far, persuasive. But it has the virtue of inviting debate on the future of American's livelihoods, and their standards of living. That's never a bad subject in a presidential campaign.