More than 120,000 Americans are expected to turn out for the Iowa caucuses to help select the Republican candidate for the presidential race.
Polling stations have been set up at more than 800 sites, including schools and libraries, across the state to choose who will stand against President Barack Obama in the November election.
Iowa is regarded as a key state in the selection process, because it holds the country's first nominating contest for eliminating some of the hopefuls and, sometimes, picking surprise contenders from among the rivals.
With six main candidates running in Iowa, only three or four will survive to fight in further state primaries. The first will be held later this month in New Hampshire, South Carolina and Florida.
Former governor of Massachusetts Mitt Romney, 64, is the frontrunner. Expecting to be named as the Republican nominee, he confidently told a rally in Marion, Iowa: "We're going to win this thing."
Ignoring rival contestants, he focused his attack on 50-year-old Obama. "I want to see America united," he said. "I've watched a president who has become the great divider, the great complainer, the great excuse-giver, the great blamer.
"I want to make America the best place in the world for small businesses, for job creators."
He is closely followed by Ron Paul, a 76-year-old libertarian Texas congressman who is popular with Iowa conservatives. "There is one issue that has made America great and the issue that can answer all your questions is individual liberty," he told his supporters.
He nevertheless played down his chances of victory, telling ABC News that he could not imagine himself in the White House.
Rick Santorum, a 53-year-old former senator from Pennsylvania, is also recognised as having momentum in Iowa after staging an 11th-hour surge in opinion polls.
"Get folks and bring them to the caucus with you," Santorum told supporters in Perry, Iowa. "We're not looking for an executive officer for this country; we are looking for a commander in chief."
A final poll showed Romney appealing to 22.8 percent of the voters, 1.3 percent above Paul, while Santorum was in third place with 16.3 percent.
Santorum was criticised by the other candidates for supporting spending bills while he was in the upper chamber.
Meanwhile, governor of Texas Rick Perry, 61, and Michele Bachmann, 55, a congresswoman from Minnesota, are fighting for fourth place to keep their chances for the presidency alive.
Newt Gingrich, 68, a former speaker of the House of Representatives, predicted that he will not win, but pledged to stay in the race nonetheless.