In analyzing Rep. Richard Gephardt's appeal to American voters -- at least in Iowa and New Hampshire -- reporters covering the campaign and the professional ''pols'' agree that the Missouri congressman has tapped into, and benefited from, ''a vein of discontent.''

Thus, my Washington Post colleague Tom Edsall, writing from Manchester, N.H., noted that voters respond favorably to Gephardt's ''semiprotectionist, anticorporate message.'' He says it is in large part a voter reaction to Reaganomics, which benefited the rich while holding level or even depressing income at the middle or lower rungs of the economic ladder.

''Massive dislocations from the shift from manufacturing to service employment, and downward pressure on wages, appear to have made many among traditional Democratic constituencies highly receptive to an economically populist message,'' Edsall wrote.

The trouble is that even if one accepts the notion that Gephardt has diagnosed America's economic illness correctly, his prescription for a cure is all wrong. In an effort to abort the shift away from blue-collar employment -- which has been going on for the last 60 or 70 years -- Gephardt would guarantee that we fall further behind Japan and some West European countries that have been beating the pants off us.

In effect, he would try to preserve blue-collar manufacturing jobs at the expense of accelerating true international competitiveness. If America is in an economic tailspin, Gephardt would unknowingly speed the process.

As the highly respected Peter F. Drucker wrote in Foreign Affairs (Spring 1986), that is precisely the course that Great Britain chose. And the result was the steepest industrial decline -- and the highest unemployment rate -- of any major noncommunist nation over the past 25 years.

Gephardt could learn a lot from Drucker, who disposes quickly of many of the myths that underlie Gephardt's nationalistic pitch -- for example, that America has become ''deindustrialized.''

The fact is that U.S. industrial output has not declined as a share of the gross national product. America has been exporting more industrial goods than ever before, despite an overvalued dollar that prevailed until recently.

''It is not the American economy that is being 'deindustrialized.' It is the American labor force,'' says Drucker.

What has happened is that as men and women have been displaced by machines, manufacturing output has soared, and employment in factories has declined. In the 1920s, one of every three American workers wore a ''blue collar.'' By the 1950s, the ratio had dropped to one of four. Now, it's less than one of six -- and falling.

It's the same in all other developed countries -- even more so in Japan. But only Japan has fully accepted the inevitability of the trend. Drucker estimates that by the year 2010, blue-collar jobs in America will be down to 12 million, compared with 18 million in 1986 and 23 million in 1975.

Obviously, the drop in some industries over the next 25 years will be even more drastic: the U.S. auto industry, with production up 50 percent, probably will be down to one-third of its blue-collar force 25 years from now, Drucker estimates. But robots don't vote, and Gephardt -- and America's union leaders -- would like to preserve, as much as possible, the present relationship of blue-collar workers to output.

The corporate ''establishment'' -- assailed by Gephardt -- well understands that to have a chance against Japanese (and other Asian) companies that plan on doubling production every 20 years with a cut of blue-collar jobs of up to 40 percent, they must do the same. If they don't, they will be out of business -- and there will be no payroll at all.

''Implicit in this is the conclusion that a country will have less overall employment the faster it shrinks blue-collar employment in manufacturing,'' Drucker says. Alas, as Drucker acknowledges, the harsh realities are not easy to accept for labor leaders or politicians.

But the process of blue-collar job loss is nevertheless going to go on, and will be hastened by new technological advances and by the shift in profitability from the large companies to ones of more modest size.

Gephardt is right that America has a massive economic problem, one complicated by the failures of Reaganomics. But America must prepare for a further shrinkage of its blue-collar labor force, not try to halt it. We need a compassionate social program that pays more than lip service to adjustment, retraining and education -- and I fault Gephardt for not facing up to this need. But Bush, Dole, Dukakis, Simon and the other candidates haven't tackled these issues head-on, either.

The answer to the ''vein of discontent'' won't be found by rolling back the clock to restore the smokestack industries to their former eminence. The good old days of declaring that America is first are over. The solution, as Drucker says, lies in abandoning nationalism and in finding ways of becoming more competitive in a global economy.