After months of a tumultuous campaign featuring outsiders who have upended the US political system, voters will finally have their say in Iowa - the first test of the 2016 presidential race.
The extraordinary performance of  outspoken billionaire real estate tycoon Donald Trump, Democratic socialist Bernie Sanders and anti-Washington firebrand Ted Cruz in the polls and on the campaign trail has stunned the chattering class and energised voters infuriated by business as usual in the US capital.
The result from the Iowa caucus on February 1 may well certify what many Americans have known for months: political outsiders have landed like an earthquake.
They have disrupted the traditional paths to power and heaped tremendous pressure on establishment candidates like Democratic frontrunner Hillary Clinton and Republican hopeful Jeb Bush.
Populist Trump and ultra-conservative Cruz are topping the broad Republican field, taking their battle down to the wire in Iowa, where a victory would provide a massive psychological boost heading into the next votes, in New Hampshire on February 9 and then in South Carolina on February 20.
At least so far, they are leaving traditional party candidates like Bush, New Jersey Governor Chris Christie and Ohio Governor John Kasich - all of whom tout their substantial experience - floundering in their wake.
If anger is the dominant theme of 2016, no one has tapped into it better than Trump.
His attacks on President Barack Obama’s foreign policy, opposition to immigration and savage take-downs of his Republican rivals - he called Bush, the son and brother of two presidents, a “sad sack” on Thursday - have riled up supporters.
His call for a ban on Muslims entering the United States caused a global firestorm, but it did not dent his standing atop the US polls.
Whether Trump can convert voter frustration into success in Iowa, where voters must brave winter weather and participate in a cumbersome voting process, is his real test.
On the Democratic side, frontrunner Clinton must be eyeing the Sanders challenge and wondering if she is facing a repeat of 2008, when she was upended in Iowa by a little-known Obama, who went on to win the nomination and the presidency.  
Clinton said she has worked hard to draw contrasts with the more grass-roots Sanders, who is calling for nothing less than a “political revolution” in America.
The pair have disagreements on guns, taxes and healthcare. Former secretary of state Clinton has sharpened her attacks on her main rival, insisting she has the stronger experience and judgment to be commander-in-chief.
And while Clinton supporters may have expected their candidate to waltz to the nomination in her bid to become the nation’s first female president, Sanders is gearing up for a protracted battle.
In Iowa, for candidates in both parties, it may come down to who has the best ground operation and is best able to get supporters out of the house and into the caucuses.