Front-running Republican presidential candidate Mitt Romney is now looking towards the next battle in New Hampshire after managing only the slimmest of victories in Iowa.
Just eight votes separated Mr Romney, the former Massachusetts governor, from former Pennsylvania senator Rick Santorum.
Mr Santorum shot from single-digit support in the Mid-western state to become Mr Romney's strongest competitor for the nomination.
The race will play out over the next six months in primary elections and caucuses nationwide before the November election. New Hampshire votes on Tuesday. "I've got a big target on me now," Mr Romney said. "I've got broad shoulders. I'm willing to handle it."
Meanwhile Michele Bachmann announced she was withdrawing from the race after finishing last in Iowa. Texas governor Rick Perry, who finished fifth, declared his intention to continue campaigning despite widespread belief he, too, would leave the race.!"
As the Iowa returns were counted, they showed a near tie among Mr Romney, Mr Santorum and Ron Paul. Hours after the polls closed, Iowa Republican chairman Matt Strawn reported that Mr Romney had 30,015 votes, with 30,007 for Mr Santorum, whose fortunes were carried by his appeal to the state's evangelical Christians and social conservatives in a state that is largely farmland and 91% white.
The count from all 1,774 precincts showed Mr Romney with 24.55% support and Mr Santorum with 24.54%. Mr Paul, who drew a younger crowd with his libertarian views, drew 21.5%.
Before his victory was announced, Mr Romney added to his already-formidable national network by announcing the endorsement of senator John McCain, who twice won the New Hampshire primary and was the Republican nominee in 2008.
While the nominal winner in Iowa, Mr Romney's campaign has failed to catch the imagination of a Republican base there or nationwide. Those voters have grown ever more conservative in recent years and view him with grave distrust for having taken moderate positions in the past. But he is the favourite of the party establishment, which sees him as the candidate best equipped to defeat President Barack Obama.
Even before his victory was announced, Mr Romney looked past his Republican rivals and took aim at Mr Obama. "The gap between his promises four years ago and his performance is as great as anything I've ever seen in my life," he told supporters.