A high turnout is expected among Republican voters in Iowa – the first US state to cast its choice over who will represent the Grand Old Party in the race for the White House.
Mitt Romney leads the field of seven candidates according to the latest polls but there is little difference between them, making the outcome too close to call.
The former Massachusetts governor has largely ignored the midwestern state but the changing fortunes of his rivals have seen him emerge as the slight frontrunner.
But Ron Paul, the small government, anti-war libertarian Texan Congressman enjoys widespread support in the largely rural community.
Some forecasters are expecting Texas governor, Rick Santorium to spring a suprise though as a result of his huge leap in popularity over the last few days.
Tea party favourite Michele Bachmann could be among the biggest losers if she ends up out of the top three places as the also-rans often then lack the money and momentum to push on.
ABC news correspondent Aaron Katersy told euronews why it is such a close contest: “The Republican party is divided : there are the social conservatives, the cultural conservatives, the fiscal conservatives. There is not one unifying candidate that seems to have emerged as the one to gather everyone under the umbrella.”
All of which could favour the US President Barack Obama come November when the contest becomes a straight two horse race.