DES MOINES, Iowa (Reuters) - Voters in Iowa begin the process of choosing the next U.S. president on Thursday with two close nominating races, as a new poll showed Democrat Barack Obama leading John Edwards -- with Hillary Clinton falling to a potentially damaging third.

The Reuters/C-SPAN/Zogby poll also showed Republican Mike Huckabee expanding his lead on rival Mitt Romney as the most heavily contested presidential caucus in Iowa history draws to a close.

Other polls show both races even tighter in the final hours before Iowa opens the state-by-state battle to choose candidates to succeed President George W. Bush -- a process that will climax in the November 4 presidential election.

The Democratic caucus begins at 6:30 p.m. CST (7:30 EST), with Republicans starting 30 minutes later. Results could begin to appear within an hour or two.

Candidates hit the trail on Thursday for final rallies before the evening caucuses, focusing on driving home their message to undecided voters and launching a mammoth voter turnout effort.

“We are going to prove that our campaign to stand up for the middle class and stop corporate greed in America is unstoppable,” Edwards, a former trial lawyer and North Carolina senator, told a rally for steelworkers and campaign volunteers in Des Moines.

For the winner in Iowa, the prize is valuable momentum and at least a temporary claim to the front-runner’s slot in their party’s nomination battle.

The third-place finisher in the heavyweight Democratic showdown, meanwhile, could find themselves hobbling into the next contest in New Hampshire on Tuesday.

The final Reuters/C-SPAN/Zogby tracking poll showed Obama, who would be the country’s first black president, holding a 4-point lead over Edwards at 31 percent to 27 percent.

Clinton, the former first lady who would be the country’s first female president, slipped to third place at 24 percent. The survey carries a statistical margin of error of plus or minus 3.3 percentage points.

Polling in Iowa is notoriously difficult given the unpredictability of who attends a caucus, the arcane rules and the low rates of participation -- fewer than 250,000 of the state’s almost 3 million residents are likely to vote.

For Clinton, who a few months ago was considered in some quarters the almost certain Democratic nominee, a third-place finish in Iowa would create immense pressure to win in New Hampshire next Tuesday.

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LUNCH IN DES MOINES

Clinton had lunch with her husband, former President Bill Clinton, their daughter Chelsea and her mother Dorothy Rodham in a Des Moines restaurant along with supporters.

Huckabee, the former Arkansas governor whose bare-bones campaign has steadily climbed since a second-place finish in the Iowa straw poll in August, expanded his lead over Romney in the Republican race, leading 31 percent to 25 percent.

Huckabee returned to frigid Iowa on Thursday after flying to California to appear on “The Tonight Show” with Jay Leno.

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In Burlington, Iowa, he made a veiled reference to Romney’s relatively recent switch to become an opponent of abortion rights while talking about his own unwavering belief in the sanctity of unborn life.

“People here want consistency. They want somebody who believes something ... and not because they got it from a focus group or an opinion poll,” said Huckabee, a Baptist minister whose rise has been fueled by support from Iowa’s sizable bloc of religious conservatives.

Romney, a former governor of Massachusetts who has had to justify his Mormon faith during the campaign, defended his aggressive advertising campaign against Huckabee and Arizona Sen. John McCain, his top rival in New Hampshire, during a stop at a Des Moines financial company.

“I’m going to be forthright on differences on issues we have,” he said. “Part of politics and a good campaign is defining those differences so people know where they line up.”

The 2008 campaign is the most open presidential race in more than 50 years, with no sitting president or vice president seeking their party’s nomination.

Iowa’s opening contest in the nominating battle has traditionally served to winnow the presidential field of laggards and elevate some surprise contenders.

Record turnout is expected for the Democrats, surpassing the 124,000 Iowans who participated in 2004. Republicans could challenge their record of 87,000 caucus participants in 2000.

Sub-freezing temperatures were predicted for Thursday night, when Iowans will leave their homes and join their neighbors at a community gathering spot to publicly declare their support for a candidate.

Not everyone was happy with the focus on Iowa and next week’s primary in New Hampshire.

“Defenders say both states take the process seriously, and participants can quiz contenders directly on a snowy sidewalk. Sorry, but that’s not good enough,” the San Francisco Chronicle said in an editorial, proposing a revamp of the nominating system.

(Additional reporting by Matthew Bigg, Andy Sullivan and Ed Stoddard; Editing by Chris Wilson)