MANCHESTER, N.H. -- Richard Gephardt's campaign has tapped into a vein of discontent underlying the five-year economic recovery. Whether the Missouri congressman's stands have been adopted for convenience or principle, his surge among voters provides new insight into the shape of the 1988 American electorate.

Gephardt's success in Iowa, and the strong possibility that his semi-protectionist, anti-corporate message will reverberate through much of the white, rural South, point to weaknesses within the contemporary economy that may be mined for political purposes.

In addition, poll analysis of the composition of Gephardt's support indicates that, after nearly eight years of the Reagan presidency, the definition of political conservatism needs to be rewritten. Gephardt's populist message has reached a set of voters who see themselves as conservatives: working and lower-middle-class white Democrats who in recent years abandoned their party to cast ballots for Republican presidential candidates.

Even if he falters, the Gephardt candidacy sends a strong signal: White, working and lower-middle-class resentment is no longer focused on government interference and tax burdens. Such resentment sparked angry tax revolts in states across the nation and gave momentum to the sharp shift to the right at the start of this decade. Now, instead, 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.

Gephardt's early success suggests that there may be a strong political reaction to the Reagan policies that, together with shifts in the domestic and world economies, have sharply improved the income of those at the top of the scale, while holding even or depressing the income of those on the middle and lower rungs of the economic ladder. In other words, there may, in fact, be political clout in the dry studies emanating from Congress and the Department of Commerce.

A recent study by the Congressional Budget Office, for example, showed that from 1977 through 1988, after-tax family income will decline for everyone except those in the top 20 percent of the income distribution. For those in the very top one percent, annual family income will have grown by more than $117,000 over the period. The family right in the middle, however, will see a net loss of just over $1,000 a year.

These changing income patterns coincide with a radically altered job market, altered in a way that often hurts New Deal, working-class Democrats the hardest. For the 30-year period from 1949 to 1979, manufacturing jobs grew at a rate of three million every decade, reaching a high of 26.5 million in 1979. From 1979 through 1986, however, the number of manufacturing jobs actually declined by 1.6 million, dropping to 24.9 million.

This disruption in the basic job structure of the nation gives muscle to Gephardt as his ads pound the airways declaring:

"There are those who say that to move ahead we must leave millions of Americans behind, that we must abandon those who build our cars, forge our steel and farm our land. History and our hearts tell us they are our strength and our cause . . . . Your votes will be the most powerful voice in this land. When it's over, let it be said that you changed America, and gave it back its soul." What remains untested in this new political climate is whether the constituency Gephardt is seeking to cultivate is adequate first to win the Democratic nomination and then, if successful, to form a secure base on which to build a general election coalition. Equally unknown is whether Gephardt himself is up to the task of achieving either goal, although his campaign style and confidence have blossomed over the past two months as his message has become increasingly aggressive and defiant.

"Gephardt is doing something {Massachusetts Gov. Michael} Dukakis and the other candidates are not doing: He is concentrating on a message and not just image," said Geoff Garin, a Democratic pollster who is not affiliated with any of the competing presidential campaigns.

That angry message promising to take on "Corporate America," to "vigorously enforce antitrust laws and crack down on mergers and takeovers," to "stand up for our jobs, our farmers, our seniors," is, in Garin's assessment, "really selling to the traditional working class base of the Democratic Party. The issue for these voters is: who is going to fight for them?"

The core of Gephardt's constituency is made up of men and women who are, on average, older than the rest of the population, somewhat less affluent, more blue-collar and, perhaps most unexpectedly, people who view themselves as conservatives. In the liberal-leaning universe of Iowa Democratic caucus-goers, Gephardt put together a plurality victory on the basis of supporters who identify themselves as tilting to the political right. That tilt, however, is very different from the economic conservatism of the 1980 Reagan revolution that led to the enactment of major corporate tax cuts.

"They feel they are getting the short end of the stick," said Ed Reilly, Gephardt's pollster. "Their antagonistic feelings toward corporations are based on the view that the corporations are treating American workers as a secondary interest. They don't want to punish corporations with taxes . . . . They are looking for leadership that holds corporations accountable to workers, not higher taxes. They want a government that stands for fairness for working men and women. What these people are saying is that 'I'm not in the equation anymore.'" While Reilly's assessment may be colored by the fact that he wants to promote Gephardt, it is not contradicted by pollsters for competing campaigns. William Hamilton, who has been conducting surveys for the faltering campaign of former Arizona governor Bruce Babbitt, said Gephardt's voters "are the people who left the party in 1980 and 1984, who feel alienated from the Democratic Party." Hamilton, who specializes in polling southern voters, believes that Gephardt's message will be very strong in the white, rural South where, Hamilton contended, Gephardt "is going to kill" Sen. Albert Gore Jr.

"Gephardt is the candidate of people who don't like liberals," Hamilton said. Although Gephardt's Iowa and New Hampshire campaign messages could not be described as conservative -- at least in the social and economic terms associated with Reagan and the right wing of the Republican Party -- most voters who supported Gephardt's candidacy in Iowa did see themselves as conservative. According to CBS/New York Times polls of attendees at the Democratic caucuses, 41 percent of Gephardt's backers were self-described conservatives and only 17 percent were liberals, with the rest either moderate or declining to answer. By contrast, supporters of Sen. Paul Simon (D-Ill.) split 21-24 between conservatives and liberals, and Dukakis' backers split 9-23.

In certain superficial respects Gephardt is courting the same traditionally Democratic voting base that Walter F. Mondale used to anchor his drive for the nomination in 1984. But for Mondale, as Garin pointed out, the strategy produced "the worst of both worlds: First he alienated the upscale Democrats in the primary process, and then he became a Democratic liberal in the bad sense of the word for blue-collar voters in the general election."

Gephardt's campaign, however, is not following the identical path. For one thing, he lacks the extensive backing of almost every major constituency group characteristic of the Mondale campaign -- although Gephardt has repeatedly modified or altered his views on subjects ranging from abortion to major weapons systems as he seeks support from varying groups. A still more important difference from Mondale is the high level of self-identified conservatives among Gephardt supporters.

Dukakis' heavy following of self-identified liberals -- together with the fact that Dukakis' support is also concentrated among upscale voters -- points to an important feature of the political landscape: Liberalism, at least in the voters' minds, has become less a commitment to traditional New Deal Democratic stands than a reflection of voters' income and class status.

The contest two days from now in New Hampshire -- one of the most prosperous states in the nation -- will provide one of the toughest tests of the strength both of Gephardt's message and his own capacity to communicate with the voters. That outcome, important as it is for Gephardt's campaign, may be still more important in shaping the structure of politics through the rest of the election.

Thomas Edsall is a Washington Post staff writer.