Ms Inevitable and Mr Invincible face Iowa test

Iowa is the battleground as the opening shots are fired on the US presidential election

Ms Inevitable and Mr Invincible face Iowa test

A framed image of Hillary Clinton at the Ready for Hillary super PAC offices in Virgina on April 3, 2015 (AP Photo/Andrew Harnik)

It's ice white in Iowa.

Snow on the ground, a bone-numbing chill blowing off the prairies. It's the toughest of tests for the White House wannabes.

This is where the election really begins. The Iowa Caucus is the first public vote, and a crucial one at that.

The Hawkeye State can be a launch pad or a landmine. It has a history of catapulting contenders towards the presidency, while bringing plenty more crashing down to Earth.

There are 15 candidates still in the race. But this week it really comes down to two - the battle between Ms Inevitable and Mr Invincible.

Hillary Clinton has looked like a shoo-in for the Democrats from the start, campaigning with an aura of inevitability. After all she's part of the Clinton Dynasty, a former First Lady and a one-time Secretary of State.

But the ghosts of 2008 still haunt her. It was then that she let a strong lead collapse under the impudent challenge of a charismatic outsider by the name of Barack Obama. He stole the Iowa caucus vote from under her nose and never looked back.

Hillary will be nervous this time round. Once again she's faced with a tricky outsider - Bernie Sanders.

The 74-year-old Senator from Vermont has been attracting real grassroots enthusiasm, particular among younger voters. He makes Hillary look even more like the establishment insider.

Donald Trump has also pitched himself as the convention-busting outsider, the blue-collar workers' champion, the billionaire man-of-the-people. And a Republican contender who exudes an air of invincibility.

No wonder he's confident. After all he's never lost an election. But that's because he's never fought one before.

He's never been tested at the ballot box.

So how will his brash, loud, provocative brand of politics go down? What if he loses? How will he react?

Texas Senator Ted Cruz is determined to find out. He's garnering plenty of support among Iowa's conservative and Christian electorate. He could yet trip up Trump.

Meanwhile, establishment favourite Senator Marco Rubio waits in the wings.

But above all this vote is a referendum on The Donald. Will they back him or stop him? Play the Trump card or twist?

On 1 February it's all eyes on the two frontrunners - Hillary and Donald.

A win in Iowa would see them gain crucial momentum and take a big step toward gaining their party's nomination.

Defeat, however, could cast doubt on Ms Inevitable's and Mr Invincible's chances.