Voters in Iowa began the process of choosing the next US president yesterday, and a new poll showed Barack Obama leading John Edwards in the Democratic nominating contest ? with Hillary Clinton falling to a potentially disastrous 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.

For the winner in Iowa, the prize is a valuable shot of 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 severely wounded heading into the next contest in New Hampshire on Tuesday.

The final Reuters/C SPAN/Zogby tracking poll before Iowa?s caucus yesterday showed Obama surging to a four-point lead over Edwards at 31 % to 27 %. Clinton fell to third place at 24 %.

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

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

Iowa opened the state-by-state battle to choose candidates in the November election to succeed President George W. Bush, and traditionally has served to winnow the field of laggards and elevate some surprise contenders.