Clinton for her part was battled into a virtual tie with rival Bernie Sanders, as Iowans held the inaugural vote of the 2016 White House race.

While the Clinton campaign claimed victory in a middle of the night statement, the Iowa Democratic Party placed her slightly in the lead but said there were still outstanding results in one precinct.

"The results tonight are the closest in Iowa Democratic caucus history," party chair Andy McGuire said in a statement.

Iowans flocked to churches, school gymnasiums and libraries to be the first voices officially heard in the boisterous months-long nominating process that leads to Election Day on November 8.

"To God be the glory!" exclaimed Cruz, claiming victory with 27.7 percent of the vote and staking his claim to be the new standard bearer of the right.

"Tonight is a victory for courageous conservatives across Iowa and all across this great nation," he said, clearly pleased that his heavy investment in campaigning in the deeply conservative state had paid off.

And just like that, many of the candidates including Trump and Rubio pulled up stakes, whisked away on their jets to New Hampshire where they will hit the ground running Tuesday.

Cruz will be aiming to capitalize on his sudden momentum while Sanders will be returning to what can safely be described as his home turf, and the potential to land a hammer blow against the former secretary of state and her dreams to be America's first female commander in chief.

Trump, who endured a tough night for a brash real estate magnate whose very brand is built on the concept of winning, will need to prove whether he can win in New Hampshire, where he holds a clear lead in the polls.

But his modest tally in Iowa -- just above 24 percent -- in the first vote since months of unprecedented wall-to-wall media coverage of Trump, raises serious questions about whether showmanship can be a winning presidential strategy.

A second hiccup, in New Hampshire, would spell political disaster for the billionaire.

"He's the big loser tonight," David Redlawsk, a professor at Rutgers University who was in Iowa for the caucuses, told AFP. "Iowa didn't quite say you're fired, but it was certainly not ready to hire him."

Trump sought to brush off his loss, saying he was "honored" to finish as he did after being given no chance to win Iowa at the outset.

"I was told by everybody, 'Do not go to Iowa. You couldn't finish in the top ten'," he told supporters. "I said 'I have to do it'."

Rubio, whose star has risen in recent weeks, tried to capitalize on a strong showing and his status as the top mainstream Republican. He earned more than 23 percent, according to the nearly complete party results, essentially confirming that the Republican battle is no longer just a two-man race.

"Tonight we have taken the first step but an important step towards winning this election," said Rubio, clearly over the moon about finishing just one point behind The Donald.

Iowa Democrats also showed their doubt in their party frontrunner, with Clinton in a dead heat with Sanders, who has railed against Wall Street and money in politics.

With 99 percent of precincts reported, Clinton was on 49.9 percent and Sanders 49.6 percent.

Despite the razor thin margin, Team Clinton was declaring victory.

Sanders disagreed.

"What Iowa has begun tonight is a political revolution," said Sanders, a Vermont senator, as he focused on building a movement around campaign finance reform and a more equitable society.

"So you guys ready for a radical idea? Well, so is America," Sanders said to cheers.

Clinton had been looking to lay to rest the demons of 2008, when she lost in Iowa to now-President Barack Obama, and pursue her quest for history by dealing a solid blow to her upstart challenger.

But she now faces another tough battle in Sanders' backyard, New Hampshire.

Clinton may be seeking to merely tread water for this next week, until the race shifts to South Carolina and Nevada, where she has commanding leads in the polls.

After that, there are several key votes in southern states, where Redlawsk said Sanders may have a tough time appealing to the region's Democrats, who are traditional more conservative.

For many long-shot candidates, Iowa has spelled the end of the road.

Republican Mick Huckabee announced he was suspending his campaign, as did Democrat Martin O'Malley. (AFP)