US elections 2012: the state of play

We're halfway through the contest to decide who will to challenge Barack Obama in this year's US presidential election.

But even though there's a clear frontrunner, it's still not certain when Mitt Romney will be able to claim the prize. With polls taking place today in Alabama, Mississippi and Hawaii, here's a guide to the current state of play in the race for the White House.

Mitt Romney campaigning in MississippiAP Photo, Rogelio V. Solis

What's been happening?

Elections have been taking place in US states since the start of the year to choose a Republican candidate to challenge President Obama in the general election in November.

These elections have taken the form either of an informal caucus, organised by the Republican party, or a formal primary, run by local government.

There are four main contenders in the race.

They are Mitt Romney, former governor of Massachusetts; Rick Santorum, former Pennsylvania senator; Ron Paul, a Texas congressman; and Newt Gingrich, former speaker of the House of Representatives.

How many states have voted so far?

To date, 21 of the 50 US states have held polls.

And who has won what?

Romney has come first in 13 states: New Hampshire, Florida, Nevada, Maine, Arizona, Michigan, Washington, Massachusetts, Ohio, Vermont, Virginia, Alaska and Idaho.

Santorum has come first in seven: Iowa, Colorado, Minnesota, Oklahoma, North Dakota, Tennessee and Kansas.

Gingrich has come first in two: South Carolina and Georgia.

Paul has won no states so far.

How is an overall winner decided?

Each candidate is awarded a number of delegates corresponding to how well they have done in each state. For example, although Santorum came first in Tennessee, he picked up enough votes only to win 29 of the state's 55 delegates. Romney came second and won 14, while Gingrich came third and won nine.

The first person to receive a total of 1,144 delegates wins the nomination.

How many delegates have each candidate received so far?

Romney is out in front with 438. Santorum is second with 216, Gingrich is third with 104, and Paul is last with 48.

Is Romney on course to reach the 1,144 total?

In theory, yes. Barring an unexpected upset or a scandal that could damage his campaign, Romney will eventually collect enough delegates to win the nomination and be formally anointed the Republican presidential candidate in the autumn.

Rick Santorum campaigning in AlabamaAP Photo, Eric Gay

So why are the other candidates still in the race?

Because they know that Romney is not the first choice of most Republican voters, but cannot agree among themselves as to who has the best chance of defeating him. They are hoping for the sort of upset that will do enough damage to Romney's campaign to allow them to at best become the new frontrunner, and at worse pick up enough support to influence the kind of policies Romney adopts as presidential candidate. There is an additional factor as well: Santorum, Paul and Gingrich all dislike Romney. This battle is a very personal one.

Who's likely to do well in the next round of elections?

Not Mitt Romney. Alabama, Mississippi and Hawaii are all holding ballots on 13 March. The latest opinion polls in Alabama suggest the race is too close to call, with either Gingrich or Santorum in with a chance of coming first. In Mississippi Gingrich is believed to have a slight lead over his rivals. Romney is predicted to win Hawaii.

How many delegates are up for grabs in this round?

A total of 110: 50 in Alabama, 20 in Hawaii and 40 in Mississippi.

What would be a good result for Romney?

A win in Hawaii along with victory in either of the two other states. This would show that Romney will have made something of a breakthrough in the south of the US, where up to now he has not performed well.

And a bad result?

Thumping defeats in both Alabama and Mississippi.

Where are the next polls due to take place?

The Illinois primary takes place on 20 March; Romney is tipped to do well here. Louisiana holds its primary on 24 March, which Santorum may win. Then in April there are primaries in most of the New England states. This could be the point when one of Romney's rivals drops out.

When are we likely to know for sure who will be the Republican presidential candidate?

It might not be until the party's national convention on 27 August in Tampa, Florida. This is when all of the delegates will be asked to formally register their support for one of the candidates.

There's still a very long way to go.