Can anyone stop the Dean machine?

You've got to hand it to Iowa voters: Their first-in-the-nation voting status in presidential election years comes at a price. After enduring thousands of political ads and dinnertime calls from pollsters, they'll brave the cold on Jan. 19, dragging themselves out to public schools and Legion halls across the state for three hours of bickering and balloting.

When it's all over, they might well anoint the Democratic nominee, end a storied career-or reshuffle the political deck entirely.

According to the latest polls, Iowa is a three-way contest between former Vermont Gov. Howard Dean, Missouri Rep. Richard Gephardt, and Massachusetts Sen. John Kerry. But in the horserace of presidential politics, the exact order of the top three-win, place, or show-will make all the difference.

For Mr. Gephardt, anything less than first place will be seen as a major defeat. He won the Iowa caucuses in 1988, and he has widespread name recognition after 27 years as a congressman from a neighboring state. He was counting on an easy win to propel him to frontrunner status, but that was before Dr. Dean's rise from obscurity.

Now Mr. Gephardt finds himself in second place in most polls, and that could spell doom for his campaign. In a crowded Democratic field where many candidates espouse the same views, Mr. Gephardt's main attraction has been his electability. If, as most analysts predict, the general election is decided in the big industrial states of the Midwest, Mr. Gephardt's impeccable union credentials would make him the ideal candidate to defeat President Bush. But if the Midwestern congressman can't win even in Iowa, he'll lose the one distinction that could separate him from the pack.

A loss for Mr. Gephardt in Iowa would likely be the end of his 32-year political career. He's not running for reelection to the House, and he doesn't have the money to stay in the presidential race without a big boost on Jan. 19. He has spent more than 80 percent of his total ad budget on the Iowa airwaves, leaving precious little for New Hampshire and beyond.

Dr. Dean has only slightly less to lose. If he wants to avoid an expensive, protracted, and divisive nomination fight, he needs back-to-back wins in Iowa and New Hampshire to force his opponents out of the race. His out-of-the-blue rise to prominence has taken on an air of inevitability, so an early setback in Iowa could knock the wheels off his bandwagon. Rivals like Gen. Wesley Clark and Sen. Joseph Lieberman, who opted out of the Iowa caucuses altogether, are gambling that a stumble by the frontrunner will open up the field and give them a shot in more conservative states like South Carolina, Arizona, and Oklahoma, which hold their primaries on Feb. 3.

While Mr. Gephardt and Dr. Dean fight desperately to avoid second place, Mr. Kerry covets that designation. After months of languishing in the single digits in most polls, he seems to be surging in Iowa and is now within striking distance of Mr. Gephardt. A surprise second-place finish would be the biggest story of the night and could bring his campaign back from the brink.

In trying to woo undecided voters, Mr. Kerry has positioned himself slightly to the right of his two main rivals. Both the Gephardt and Dean camps have called for a repeal of the president's middle-class tax cuts, for instance, while Mr. Kerry says he would repeal only those cuts that benefit the wealthy. By exploiting the divide between the Dean latte liberals and the Gephardt labor liberals, Mr. Kerry may be able to find enough votes to pull off the upset of the evening.

Still, polls in Iowa are notoriously unreliable and the caucuses consistently unpredictable. That's because only a few voters bother to attend them-less than 10 percent of eligible Democrats showed up in 2000-and those who do often change their votes before the night is done. Historically about one-third of voters reach their polling places with an open mind and the other two-thirds can change their minds throughout the evening.

Unlike primary election states, where voters just mark a ballot and go home, Iowa's caucuses are a tedious, time-consuming affair. They start at 6:30 p.m., when voters gather in meeting rooms in every precinct around the state. There's no such thing as a secret ballot; every voter has to publicly declare a preference. Often, that's done by dividing the room into groups: Gephardt supporters in that corner, Dean supporters over by the water fountain.

Any candidate who fails to gain 15 percent of the vote in a precinct is declared not viable, and his supporters then have the opportunity to join another group. After 30 minutes of dealing and debating, the groups form again, resulting in a final tally for each candidate. (Sometimes the supporters of a nonviable candidate can't bring themselves to back anyone else on the ballot, resulting in a large number of votes for "undecided.")

The biggest loser is likely to be North Carolina Sen. John Edwards. He has spent heavily and campaigned actively in the state, yet never reached 10 percent in the polls. That means he'll fail to break the 15 percent barrier in many precincts, and his supporters will have to look elsewhere.

John Kerry, as the most moderate of the three major candidates, might be an attractive alternative, boosting his hopes for a second-place finish. Gephardt strategists, however, insist that their candidate is a comfortable fallback for Iowa voters who identify with his Midwestern persona. They believe they can pick up enough support among vote-switchers to pass the frontrunner and defy the pollsters; indeed, it's defy-or-die time.