Races close as Iowa kicks off White House campaign

By John Whitesides, Political Correspondent

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.