Rupert Cornwell: As the first battle ends, which of these Republicans can win over the nation?

Of Romney's rivals, only Ron Paul has a genuine national organisation, built up in previous runs

To paraphrase Winston Churchill's words about the battle of El Alamein and its place in the Second World War, this month does not signify the end of the race for the Republican presidential nomination, nor even necessarily the beginning of the end. But it is very much the end of the beginning.

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Within barely three weeks the nature of the campaign will change completely. Retail politics – where candidates go from small town to small town and meet individual voters in the flesh – may be the stuff of yesterday's Iowa caucuses, next Tuesday's primary in New Hampshire and, to a lesser extent, 21 January's South Carolina primary. But after that, the campaign goes wholesale.

For months, Iowa and New Hampshire, two small and demographically unrepresentative states, accounting between them for less than 1.5 per cent of the US population, have hogged all the attention. But thereafter the focus shifts to big states and big media markets, rich in delegates, where candidates deliver their message to voters not in person on the stump, but through costly TV advertising and a ground operation. What counts is not how many hands they shake, but the money, and above all the organisation, at their disposal.

Take Rick Santorum, who according to polls was surging in Iowa, his reward for 200-plus days he has spent in the state, visiting every one of its 99 counties. But the former Pennsylvania Senator won't be able to repeat those tactics again – he simply won't have the time.

Belatedly, donors are opening their wallets; Mr Santorum claims he has raised more money in the last six days than in the six previous months. But momentum, and the money which follows, will not alone suffice. Mr Santorum must now rush to set up campaign infrastructure operations in states where he has barely set foot. Only on Monday did he air his first ads in New Hampshire and South Carolina – and the experience of Mike Huckabee in 2008 underlines the difficulty of his task.

Four years ago, Mr Huckabee mobilised Iowa's social conservatives to deliver a shock defeat to Mitt Romney. But Mr Huckabee would prove no match for the better-known, better-organised and better-financed John McCain, who wrapped up the nomination by March.

For Mr McCain four years ago read Mr Romney now. The first big primary comes in Florida. Even as their man was staging a final blitz in Iowa, the Romney campaign was mailing absentee ballot forms to potential supporters in Florida ahead of the 31 January vote. Mr Santorum cannot hope to match that – indeed no other 2012 Republican can.

Of Mr Romney's six rivals, only Ron Paul has something approaching a genuine national organisation, thanks to a devoted following built up in earlier presidential runs. In terms of money, probably only the Texas governor Rick Perry can match Mr Romney. But in 2012, all is not lost for the upstart outsider. For one thing debates, more numerous than ever this year, will provide a continuing free platform. Both the primary calendar and the delegate allocation system are kinder too. After Florida comes a four-week interval before the next important contests in Arizona and Michigan on 28 February, offering a chance to get a national infrastructure in place.

Moreover delegates in early primaries will be awarded proportionally, rather than on the winner-takes-all basis used previously. This means no candidate can wrap up the nomination by "Super Tuesday" on 4 March, when 10 states, including Ohio, Georgia and Massachusetts, hold primaries and caucuses.

Instead, the primary season has been stretched out: Texas, the second-most delegate-rich state, holds its primary on 3 April, while New York, the third, votes on 21 April. California, the largest prize of all, with 172 delegates, as well as New Jersey, votes only in June.

As a result only 700-odd convention delegates will have been chosen up to and including Super Tuesday, far short of the 1,143 needed to nominate. In theory a brand new candidate could even enter the race next month, and still have time to get on the ballot in most states.