DES MOINES, IOWA -- Rep. Richard Gephardt of Missouri has broken out of the crowded Democratic presidential pack in two ways: as belated front-runner in Monday's Iowa caucuses and by delivering a message tougher and more clearly focused than any other candidate's.

The two breakthroughs are directly related. Gephardt's sudden rise from apparent oblivion was made possible by a populist appeal to the economic losers of this deflation-ravaged state, an appeal different in kind from the compassionate liberalism of his competitors.

Those rivals scoff at Gephardt, House Democratic Caucus chairman and House Ways and Means inside operator, posing as a prairie populist. Nevertheless, if it works here Monday night as well as is promised, Republicans may be facing a class-conscious challenge in November.

Assuming his organization can get out his voters, Gephardt could be much farther ahead of Gov. Michael Dukakis and Sen. Paul Simon than is publicly acknowledged. If this were a primary instead of a caucus, Gephardt operatives say privately, there would be no doubt. Iowa old-pro Ed Campbell, a Gephardt supporter, says the congressman's management of the House defeat of contra aid clinched victory here.

Yet one month ago, Gephardt was consigned to the Iowa scrapheap, running in fifth place (with 6 percent in one poll) in a state where he must do well. His revival is attributed by many insiders to an organization he painstakingly built over two years, taking hold last month when Gary Hart's comeback flopped.

But that is the political insider's emphasis on mechanics over issues. Gephardt's breakthrough coincided with his new television spots, now famous for threatening South Korea with retaliatory tariffs, pricing their autos here at $48,000. Media consultants Robert Shrum and David Doak told Gephardt he could not win every voter and should shed his legislator's impulse for compromise.

That Gephardt has taken this advice is demonstrated in his new basic campaign speech. Always the most disciplined of candidates, he has refined his stump presentation to a fine point that bears little resemblance to his soft rhetoric of last autumn. Speaking at Council Bluffs the night before the contra debate, he inspired spontaneous cries of ''Give 'em hell, Dick,'' which could not have been imagined last year.

''Isn't it time we had a president who stood up for American business and American workers?'' he asks. Besides his now familiar promise to demand of the South Korean government, ''How they're going to sell Hyundais in America for $48,000,'' he declares, ''I want to tell OPEC where to go.'' He attacks ''The big grain companies, the food companies who don't want a change in foreign policy'' and ends by roughing up Wall Street Journal editorial writers.

The appeal to workers and farmers drowns out Simon's perpetual plea ''for a government that cares'' and Dukakis' promise of managerial expertise. While protectionist, Gephardt's message is mostly in the left populist tradition. The economic losers in Iowa are told their woes come from big business and foreigners.

Were this spoken in the rural southern drawl of George Wallace instead of Dick Gephardt's Midwestern urban modulation, it would sound more threatening. ''It would really be scary, if Gephardt believed what he was saying,'' Greg Schneiders, consultant to Bruce Babbitt's campaign, told us.

Stigmatizing Gephardt as a pinstripe populist is the main hope for stopping him here. Word of mouth denunciations of him as an opportunist are publicly echoed by Babbitt. Simon, incorrectly, has assailed him for tardy animosity toward the contras, and is running radio commercials detailing Gephardt's flip-flops from conservative to liberal on key issues once he started running for president.

At Drake University (continuing his quest for upscale Democratic voters), Dukakis criticized ''waving an angry fist at Korea or Japan'' but did not mention Gephardt by name. With time running out, the net impact of all this is minimal.

Gephardt's deeper vulnerability is whether he can carry his appeal to economic losers to New Hampshire, where winners abound today. His advisers think there are sufficient voters everywhere who are desperately afraid of losing in the current economy. That should give pause not only to Gephardt's Democratic opponents but to any centrist Republican presidential nominee.