Hillary Clinton declared victory over Bernie Sanders in a razor thin contest between Democratic candidates, while arch conservative Ted Cruz scored an upset victory over Republican frontrunner Donald Trump as Americans cast their first votes in the US presidential race.
"Hillary Clinton has won the Iowa Caucus," the Clinton campaign said early Tuesday morning after the first nominating contests in Iowa gave her 22 delegates to Vermont Senator Bernie Sanders' 21 delegates - with one delegate left undecided.
The state's Democratic Party waited for the full results before announcing a victor. But its figures showed that Clinton appeared to have an upper hand in the remaining count.
"As I stand here tonight, breathing a big sigh of relief," Clinton told supporters at a Des Moines rally. "I will keep doing what I've done my entire life. I will keep standing up for you. I will keep fighting for you."
It was an apparent reference to her 2008 loss where a disastrous third-place showing in Iowa that then Senator Barack Obama won, unravelled her first presidential run.
For Sanders, the close outcome was a cause of celebration too, showing as it did his remarkable rise against the formidable Clinton machine.
"Nine moths ago when we came to this beautiful state, we had no political organization, we had no money, we had no name recognition and we were taking on the most powerful political organization in the United States of America," he said.
On the Republican side with Trump failing to translate his formidable media presence into votes, Cruz who spent months touring the state and reaching out to evangelical voters, emerged as a formidable contender in the delegate-rich, Southern states holding primaries in the coming weeks.
As he claimed victory, Cruz fired immediate shots at both Trump and the party elites who disdain him.
"Iowa has sent notice that the Republican nominee and the next president of the United States will not be chosen by the media, will not be chosen by the Washington establishment," he said.
As of early Tuesday morning with 99 percent of the vote reported, Cruz had 28 percent of caucus attendees tallied, followed by Trump with 24 percent and Florida senator Marco Rubio with a surprisingly strong 23 percent.
But Cruz may have a tougher time in New Hampshire which holds the first inthe nation primary on February 9. Trump leads in the polls there and Rubio's stronger-than-expected showing in Iowa makes him a more formidable foe.
Trump delivered a short but gracious speech, saying he loved Iowa and vowed to go on next week to win the New Hampshire primary.
"We will go on to get the Republican nomination and we will go on to easily beat Hillary or Bernie," Trump told supporters. "We finished second, and I have to say I am just honoured."
Rubio's strong showing could set him up as the best placed potential establishment candidate to take on "outsider" challengers Cruz and Trump, observers said.
"This is the moment they said would never happen. For months, they told us we had no chance," a jubilant Rubio said.
After his total washout, former Maryland governor Martin O'Malley dropped out of the Democratic race. So did Arkansas' former Republican governor Mike Huckabee.
A loss in Iowa does not mean the end of the road for a contender though a victory gives a candidate a momentum ahead of the primary contests in the remaining 49 states.
WHEN TRUDEAU MET CRUZ: HOW CANADA'S PM ONCE DEBATED THE IOWA CAUCUS WINNER
Here's a lesser-known connection between the winner of this year's first U.S. presidential contest and the home and native land of his birth: Canada's prime minister recalls debating Ted Cruz.
The Calgary-born, Texas-bred senator who stormed out of the gate Monday with a win in Iowa's Republican caucuses was once a stellar university debater, ranked No. 1 in the U.S.
Cruz's opponents included a contemporary from Montreal — a prime minister's son and prime-minister-to-be who decided to get involved with the debate club at the urging of friends at McGill University.
Justin Trudeau first publicly referred to that old encounter during a speech early last year when he was still leader of Canada's third party, and the senator had yet to announce his presidential bid.
Trudeau told a group of fellow McGill alumni that he'd dabbled in debating and had once gone up against Cruz in a tournament at Yale, adding dryly: "He hasn't changed very much."
He elaborated slightly in an interview with The Canadian Press last June. By then, Cruz had announced his presidential bid but Trudeau was still in opposition.
While the details are a bit hazy, Trudeau recalls an opponent who came prepared to dominate — and, apparently, Cruz did.
''If I recall correctly it was about an obscure monetary policy element that he had done an awful lot of research on," Trudeau said in that pre-election interview.
"His poor opposition had no real capacity to rebut. It was a focus very much on winning the debate, rather than on any sort of fair chance to have a good and robust debate. But that's the way university debating was played at that particular moment.''
If Trudeau lost to Cruz, he wasn't alone.
Before embarking on a law career where he argued nine cases before the U.S. Supreme Court, Cruz was the star of Princeton's legendary debate team. He was not only the 1992 U.S. college debater of the year, but his team was also ranked No. 1.
A New York Times profile describes an aggressive debating style that occasionally grated on judges, but ultimately earned Cruz numerous victories still commemorated on a plaque at Princeton.
Alas, visual evidence of that Cruz-Trudeau encounter appears lost to history.
When asked, neither Trudeau, his McGill friend Gerald Butts nor the debate clubs at Yale, McGill, and Princeton have video of that clash between future political heavyweights.
In his autobiography, Cruz described how his obsession with debating temporarily hurt his grades. He exasperated his teammate-and-roommate, forcing him to spend hours analyzing minute details of each contest.
They'd get home at midnight from a tournament and then: "I insisted that David (Panton) and I go up to our room, sit down, and assess our performance so we could learn from our mistakes," Cruz writes in "A Time For Truth."
"We learned from these discussions, which would extend till three or four in the morning, which would often prompt David to protest, 'Enough already! This is madness.' But the madness paid off."
An ex-aide to President Barack Obama concurs: Cruz was tough competition.
"He was at Princeton, I was at Yale. So we used to debate pretty much every week," Obama's ex-economic adviser Austan Goolsbee told Fox last week.
"He's a very, very good debater. Now the world knows that."
After watching the Texas firebrand in a Republican presidential debate last month, Butts sent a nod to Goolsbee via tweet: "Cruz's debating style is exactly the same as it was 25 years ago."
