DES MOINES, Iowa (Reuters) - Democrats in Iowa will cast the first votes in the 2004 presidential race on Monday, with polls showing John Kerry holding a slight lead in a tight four-way dash to the finish.

Kerry, Howard Dean, Richard Gephardt and John Edwards were all within striking distance of victory in what could be the closest race in the history of Iowa's caucuses, the first major step in choosing a Democratic challenger for President Bush in November.

A Reuters/MSNBC/Zogby poll released on Monday showed Kerry expanding his slim lead over Dean to 3 points, pulling ahead 25 percent to 22 percent. Edwards moved into third place at 21 percent and Gephardt in fourth at 18 percent.

At least 100,000 Democrats were expected to brave sub-freezing temperatures on Monday night to attend one of nearly 2,000 local precinct caucuses around the state and publicly declare their support for a candidate.

The latest poll found 9 percent of likely caucus-goers still undecided, leaving room for momentum swings in the last hours of the race.

A loss for Dean, the front-runner who scored a series of big endorsements in recent weeks, would open the door for other candidates and turn what was shaping up to be a quick Dean victory into a dogfight.

The former Vermont governor declared himself "ripping, roaring and ready to go" and shrugged off his drop in the polls.

"If we win tonight, it's going to be very difficult to stop us -- the ultimate comeback kid -- and then we will have gotten all this stuff behind us," he said.

A win for Kerry, a senator from Massachusetts, would be a huge boost to what had been a struggling campaign. But Gephardt, the congressman from neighboring Missouri and one-time leader in Iowa polls, had the most to lose. His campaign could be over with a loss in the state.

"I always knew it would be a tight race, Iowans are tough judges," Gephardt said. At a rally for union workers outside Des Moines, he urged them to "go to every house" and bring out supporters.

Kerry and Edwards, a first-term senator from North Carolina, surged as voters took a fresh look at the candidates after the holidays, evaluating which one had the best chance to beat Bush in November and responding negatively to an exchange of harsh attack ads by Dean and Gephardt.

TURNOUT CRUCIAL

Turnout was crucial, and Dean and Gephardt built voter mobilization efforts unrivaled in Iowa's history, with Kerry not far behind, according to local Democrats.

All four campaigns sent thousands of volunteers into neighborhoods across Iowa to hunt for voters and get them to the caucuses. Dean had more than 3,000 out-of-state volunteers knocking on doors, and Gephardt's army of union members had more than 4,000 people on the street.

The public nature of the caucuses, in which participants gather to declare their support, places a premium on campaigns that find the most committed caucus-goers. In 2000, about 60,000 Democrats participated.

Most of the candidates spent the day rallying their workers and looking to sway undecided voters before descending on Des Moines to wait for results.

On Sunday, they crisscrossed the state to drum up support.

Dean took a detour to visit former President Jimmy Carter in Plains, Georgia. Carter offered words of praise for Dean's opposition to the Iraq war, but kept his promise not to endorse anyone in the Democratic race.

On his return to Iowa, Dean was joined on the campaign trail for the first time by his wife, Judy Dean, who has remained in Vermont to run her medical practice and care for their teen-age son in high school.