The first caucus could reinvigorate the party or unwittingly hand a gift to the Democrats
Mitt Romney wins
Victory would give him a chance of wrapping up the nominating process by the end of January. He has a double-digit poll lead in the next race, New Hampshire, on 10 January. He faces a tougher time in the one after that, South Carolina, on 21 January, but even second place would do. Gingrich leads in Florida, to be held on 31 January, but he has been fading in the polls and Romney's huge funds and better organisation might help overtake him. If Romney takes Iowa, New Hampshire and Florida, the others might bow out and leave him free to concentrate on Obama.
Ron Paul wins
Problematic for establishment Republicans, who need to win over the centre ground to take the White House. The Democrats would jump in to portray the Republicans as whacky and extremist. Win or lose, Paul has the organisation and money to keep going all the way to the Republican convention in August. Proportional representation, not in place in 2008, encourages him to keep going, accruing delegates for the convention. He does not have enough support to win the nomination but, with a bloc of delegates, he could try to wring concessions from the eventual nominee, or at the very least have a big public platform to espouse his views.
Rick Santorum wins
Another headache for the Republicans, a rerun of 2008 when Mike Huckabee won in Iowa but failed to win New Hampshire, South Carolina or Florida. Santorum has devoted all his resources to Iowa and has no organisation in place elsewhere. Like Paul, the Democrats would view a win for him in Iowa as a gift, a chance to portray the Republican party as extremist. A win for Santorum would also renew questions about whether Iowa should be holding the first caucus, given the untypical make-up of the voters and its failure to pick eventual winners.
Newt Gingrich wins
Chaos in the Republican party. He is suffering a poll slump but a win in Iowa could provide him with the momentum to take South Carolina, where he still enjoys a lead. Gingrich, according to the polls, is also the frontrunner in Florida. Wins in Iowa, South Carolina and Florida could result in a drawn-out battle, state by state, month after month, as Romney would not bow out of the race.
Rick Perry manages to avoid last place
Like Gingrich, he will stay in for New Hampshire, South Carolina and Florida. He has fought a totally inept campaign but could be a slow burner, with millions at his disposal as he tries to revive a campaign wrecked by disastrous debate performances, including one incident where he apparently could not remember his own policies over which government departments he wanted to cut.
Michele Bachmann is last
She bows to the inevitable and decides to drop out of the nomination race, though she has said she intends to fight on with her campaign, bypassing New Hampshire, and heading straight to South Carolina.