Mr Paul spent years building a grassroots network of support in Iowa and has won a serious and passionate following, based largely on his anti-government, anti-tax, and anti-spending message.
Another candidate making a late, but serious, threat is Rick Santorum, who for months barely registered on polls but has seen a last-minute surge at just the right time.
Mr Santorum, known for his Christian conservative positions and anti-abortion crusade has fought hard for a strong finish in Iowa, hitting every county in the state over the past few months despite very little money or organisation.
The state's large evangelical bloc is splitting support among Mr Santorum, former House Speaker Newt Gingrich, Texas Governor Rick Perry and Minnesota Congresswoman Michele Bachmann.
All three deeply conservative candidates, rose to the top of the Republican field over the past months as alternatives to Mr Romney, but they quickly faded under closer media and voter scrutiny.
A poor showing in Iowa – coming in fourth or worse – could be the end of some of their campaigns. No candidate has ever finished less than third in Iowa and gone on to win the party's nomination.
There is a seventh Republican in the race – former Utah Governor Jon Huntsman – who is not contesting Iowa and has focused almost solely on New Hampshire.