The Democrats have also differed over taxes, with Gephardt and Dean favoring a full rollback of Bush's tax cuts, and others calling for repealing just those benefiting the wealthiest Americans.
But the candidates agree far more than they disagree, assailing Bush on everything from his environmental positions to his education, health-care and foreign policies -- sometimes using almost exactly the same words.
Electability, that intangible sense of who stands the best chance of beating Bush in November, appeared uppermost in the minds of many Iowa Democrats.
"That's imperative," said 70-year-old Donna Casey of Massena, who was trying to decide between Gephardt and Kerry. "Another four years and we'll be in deep trouble."
The candidates differed substantially in their styles, with the fiery Dean leveling the harshest attacks on Bush and assailing his opponents as captives of a Washington establishment he vows to turn upside down.
That served him well for months, striking a chord among Democrats who felt others in the field were too conciliatory, and fueling a rise that established him as the clear-cut frontrunner.
Lately, however, Dean opponents have vigorously challenged his electability. That, coupled with Dean missteps and a series of unflattering media reports, seems to have dragged him back into the pack of candidates.
Handicappers still considered Dean a favorite to win Monday night, thanks to his strong organization in the state, numbering more than 2,000 volunteers by his campaign's count -- including singer Joan Jett, comedian Janeane Garofolo and three Americans who traveled from Japan. The volunteer army had a goal of knocking on 200,000 doors by Monday night.
Gephardt, who almost certainly must finish first to carry on, was trusting in the muscle of more than 1,000 labor organizers, representing 21 unions, who swarmed the state on Saturday. "They're all really putting the shoulder to the wheel to make this happen," the congressman told a crowd in Solon.
Kerry, who served in Vietnam, was relying on a built-in advantage among the state's veterans, working off research showing that as many as 90,000 of them expressed interest in the Democratic race. Saturday, he held an emotional reunion in Des Moines with a man he had saved under fire in Vietnam. The two had not spoken in 35 years.
His campaign appeared to be connecting in a way it had not previously, helped by TV ads focused on health care and a newfound passion on the candidate's part. He largely abandoned his windy speaking style for a more direct and empathetic appeal.
Edwards, with probably the weakest ground operation in the state, was counting on momentum from his strong home-stretch finish, reflected in crowds 10 times bigger than those he was seeing a few weeks ago. His campaign said 500 out-of-state volunteers had come to Iowa for the weekend, buttressing a paid staff of about 100.
Jeri Powell, 26, oversees about 30 precinct captains for Edwards in the Des Moines area. "They're using handwritten notes, door-knocking," said Powell, who moved from Asheville, N.C., to join Edwards' campaign. "It's a personal connection."
The upshot was a campaign too close to call.
"One way or another, all four are connecting with an element of the electorate," said Stuart Rothenberg, a Washington campaign analyst. "At the moment, each of those slices of the electorate are roughly the same size."
Thousands of volunteers, from inside and outside Iowa, were working the state for Gephardt, trying to coax supporters to the caucuses. Among them was Sue Dvorsky, his captain in Precinct One in Coralville, in eastern Iowa, who helped organize the Solon town meeting.
"We've got a list," said Dvorsky, 48, the wife of an Iowa state senator. "We're making calls; we're getting to doors."
But Dvorsky acknowledged the ferocity of the competition. Her best friend, she said, was doing the same thing for Kerry.
Among the senator's surrogates traveling the state on Saturday were Kerry's wife, Teresa Heinz Kerry, and Iowa's first lady, Christie Vilsack, who is set to appear in a new TV ad for Kerry. Former Sen. Max Cleland (D-Ga.) -- who lost three limbs in the Vietnam War -- was rallying veterans.
After Iowa, the candidates will square off in New Hampshire, which conducts its primary Jan. 27.
Clark and Lieberman have been campaigning extensively there while the others have focused on Iowa.
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The Iowa caucuses
The Iowa caucuses -- the first test in the presidential nomination process -- require voters to invest a few hours of their time on a winter's night. Dating back to the 19th century, the caucuses were not nationally significant until the 1970s, when Iowa's Republican and Democratic parties moved their contests to January, securing 'first in the nation' status. Here's how Iowans elect delegates at the caucuses:
When: Monday, starting at 6:30 p.m. CST (4:30 p.m. PST)