Republican Senator Ted Cruz bumped billionaire Donald Trump from the top spot and Democrat Hillary Clinton faced a fierce challenge Monday as Iowans voted in the first nominating contest of the 2016 White House race.

Republican voters in the rural midwestern state backed the ultraconservative Cruz for the party nomination, according to US media projections -- leaving Trump to battle with a resurgent Marco Rubio for second place.

Iowa Democrats also showed their doubt in their party frontrunner, with Clinton fighting neck-and-neck with left-wing populist Bernie Sanders, according to partial results.

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

Trump has dominated polls and the media for months, but his 24.3 percent vote tally, according to partial results, raised questions about whether showmanship can take him all the way to 1600 Pennsylvania Avenue.

The real estate mogul who has centered his campaign around being a "winner" tried to brush off the loss, saying he had been 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."

Cruz, who has invested heavily in ground operations, will hope to use the victory in Iowa -- he was on 27.7 percent, with 92 percent of precincts reporting -- as a springboard to the next contests in New Hampshire and South Carolina.

"What a bit of history we are seeing," the Texas senator told fired up supporters earlier Monday.

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.

With 91 percent of the precincts reporting, Clinton held 49.8 percent of the vote to 49.5 percent for Sanders.

Clinton is under intense pressure to win here before going back to New Hampshire where Sanders, who is from neighboring Vermont, holds a strong lead.

For Sanders, the prospect of more donations that could help fund a long challenge beckons.

Get out the vote

Initial reports showed high voter turnout, which experts had predicted would benefit the political outsiders who have dominated the 2016 race so far.

"This is very chaotic but I'm happy that so many people showed up," said Aaron Menick, a 21-year-old at a Democratic caucus at Drake University.

Activists in one Des Moines precinct reported turnout was three times higher than expected.

Armies of campaign volunteers have fanned out through the state in recent days, knocking on doors or manning phone banks to get out the vote, while candidates dominated the air waves with talk show appearances and a relentless barrage of campaign ads.

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

Republican Mick Huckabee announced he was dropping out of the race, and sources close to Democrat Martin O'Malley said he would follow suit.

Even as votes came in, fourth place Ben Carson was forced to deny reports he was dropping out of the race.

"Contrary to false media reports, Dr. Ben Carson is not suspending his presidential campaign, which is stronger than ever," said campaign spokesman Larry Ross.

Meanwhile Rubio, whose star has risen in recent weeks, tried to capitalize on a strong showing, at 23.1 percent -- according to partial results.

The Florida senator has emerged as the top mainstream Republican.

"Tonight here in Iowa, the people of this great state sent a very clear message," Rubio told supporters.

"Tonight we have taken the first step but an important step towards winning this election."