Republican Ted Cruz has beaten Donald Trump in Iowa's presidential nominating contest for the Republican party, the first in the 2016 race for the White House.
In the Democratic poll, Hillary Clinton and Bernie Sanders were squared off in a virtual dead heat late on Monday.
Cruz, a conservative politician from Texas, won with 28 percent of the vote against 24 percent for the controversial businessman Trump.
"Tonight is a victory for courageous conservatives across Iowa and all across this great nation," Cruz said in his victory speech.
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Iowa has given notice that the next Republican nominee "will not be chosen by the mainstream, will not be chosen by the Washington establishment, will not be chosen by the lobbyists - but will be chosen by the most incredible, powerful source ... the American people," he added.
Marco Rubio, a senator from Florida, came third with 23 percent, making him easily the leader among establishment Republican candidates.
William Schneider, a professor of public policy at George Mason University, said the candidate from Florida is "likely to go all the way".
"Republicans had a division, those who voted with their hearts voted for Cruz. Those who voted with their heads voted for Rubio because he looks like a winner," Schneider told Al Jazeera.
"At this point, the Republican power-holders are very fearful of Cruz taking the nomination because they don't think he can get elected. You're going to see pots and pots of money being thrown at Marco Rubio."
Clinton, a former secretary of state, held a lead of 49.8 percent in the Democratic race against 49.6 percent for Sanders, a self-styled democratic socialist senator from the state of Vermont, with 99 percent of the precincts reporting.
"This is astonishing, really. It was really Hillary Clinton's race to lose," Al Jazeera's Kimberly Halkett reported. "When Bernie Sanders launched his candidacy, Clinton was some 50 points ahead. Now it is virtually the same number."
Halkett said while a win in Iowa of the Democratic or Republican caucuses doesn't always point to an eventual win of the nomination, it provides important momentum and and drives donations for further campaigning.
"Historically a win is important for another reason too. No candidate who has finished less than third place has ever gone on to win the White House," she said.
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In the Democratic race, former Maryland Governor Martin O'Malley was considered the most endangered Democrat before the caucuses.
Sources close to his campaign told Al Jazeera that O'Malley would suspend his campaign after the poll.
On the Republican side, Mike Huckabee announced that he was pulling out of the presidential race after a poor result on Monday.
Further departures may occur after Iowa, but New Hampshire on February 9 may be more of a winnowing event, because that is where more of the hopefuls are pinning their expectations.
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Source: Al Jazeera and agencies