DES MOINES, IOWA - Texas Senator Ted Cruz won the Iowa caucuses, snatching victory from Republican front runner Donald Trump, while Democratic front runner Hillary Clinton declared victory after being locked in a virtual tie with Vermont Senator Bernie Sanders.

Conservative Republican voters in the mid-western state turned up in full force for Mr Cruz, leaving Mr Trump just barely ahead of Florida Senator Marco Rubio.

Mr Cruz gained 28 per cent of the votes, while Mr Trump followed with 24 per cent, and Mr Rubio with 23 per cent.

While Mr Trump had dominated polls and the media for months, the results raised questions about his ability to ultimately win the party's nomination.

On the Democratic side, Mrs Hillary Clinton's campaign declared victory in a statement released on Tuesday. "Hillary Clinton has won the Iowa Caucus. After thorough reporting - and analysis - of results, there is no uncertainty and Secretary Clinton has clearly won the most national and state delegates. Statistically, there is no outstanding information that could change the results and no way that Senator Sanders can overcome Secretary Clinton's advantage," said Mr Matt Paul, Hillary for America's Iowa state director.

But a shadow of doubt was also cast on Mrs Clinton's campaign, as she and Mr Sanders were neck and neck all the way to the end. With 99 per cent of the votes counted, Mrs Clinton had 49.9 per cent of the votes, while Sen Sanders had 49.6 per cent.

While Iowa only has a small number of delegates - 44 on the Democratic side, and 30 on the Republican side - who will ultimately go on to the national convention to vote for the eventual party nominee, it plays a big role in creating momentum for each candidate's campaign.

Political science professor at Iowa State University Mack Shelley said Mr Cruz's win boils down to "lots of boots on the ground and organisational support".

Also to Mr Cruz's advantage was "the heavy turnout in the north-western corner of the state which is heavily evangelical", he added.

Speaking to his supporters, Mr Cruz said his win was "a victory for the grassroots" and for "conservatives across Iowa and all across this great nation".

On the Democratic side, Mrs Clinton told her supporters, after the votes were mostly counted, that she "breathed a great sigh of relief" and would keep standing up for them and fighting for them.

A good showing in Iowa was important to Mrs Clinton as she had lost the state to President Barack Obama in 2008, and was hoping to avoid a similar situation.

Doing well in the Hawkeye state was also important for Mrs Clinton as Mr Sanders has a strong lead over her in New Hampshire, the next state where the primary voting will take place.

Acknowledging that the battle continues, Mrs Clinton said: "I am excited to get into a debate with Senator Sanders about the best way forward to fight for us and America."

Feeling the "Bern" as supporters chanted his name, Mr Sanders was all smiles as he addressed his supporters in Des Moines.

"I think the people of Iowa have sent a profound message to the political establishment, the economic establishment and the media establishment," said Mr Sanders. "Given the enormous crisis facing our country, it is just too late for establishment politics and establishment economics."

Prof Shelley said Mr Sanders could claim credit in exceeding expectations. "Sanders can spin this more in his favour. He came literally out of nowhere to run dead even with someone who was supposed to be anointed and go on to get the nomination pretty much automatically."

Moving forward, "it certainly seems to help him more than it helps her".