After a long, expensive and wearisome preamble, the story of the 2008 nominating campaign for the White House will be written in a relatively short series of pages. The Iowa caucus will be held on Thursday. The New Hampshire primary will take place a week today. The so-called “Tsunami Tuesday” on February 5 — when 20 states vote, including some of the most populous in the Union — will then probably prove decisive. This will thus be a short and spectacular campaign. Any individual and institution, inside the United States or beyond its shores, who wishes to express an opinion should speak now.

The challenges for the Republican and Democratic parties this year are distinct. The threat to the Republicans, as the party whose President presently occupies the Oval Office, is that of history. The natural pendulum of politics in the United States suggests that after eight years of one party in control of the executive, the inclination of the electorate is, usually cautiously and by a narrow margin in the electoral college, to transfer authority to its opponents. This was the pattern in 1960, 1968, 1976 and 2000. George Bush Sr bucked the trend in 1988 only to be defeated four years later. This sense of the inevitable has been reinforced by George W. Bush's low poll ratings over the past 30 months and by the Democratic victory in the House and Senate in November 2006.

That outcome, though, is far from certain. One of the reasons why the party holding power in the past has been unable to maintain command of it is that their nominee has been the serving vice-president (or, in Gerald Ford's case in 1976, a vice-president turned accidental president). That individual has then found it hard to establish his own political identity. All the Republican challengers this time have room in which to emphasise their independence from the White House.

The new Democratic Congress has not been a notable success; indeed, surveys imply its leaders are more unpopular than Mr Bush. So a Republican victory against the historical odds is not impossible. But if it is to occur, the candidate must be able to convince fellow citizens that he represents change and not virtual continuity from the Bush era, and that he is of such a calibre and quality himself that voters who dislike his party label can ignore it.

If these are the qualities demanded, then the choice of candidate becomes quite simple and not merely by a process of elimination. There are five realistic nominees available: Rudy Giuliani, Mike Huckabee, John McCain, Mitt Romney and Fred Thompson. Only one of them can really portray himself as an agent of “change” or the sort of man whose record would inspire respect in the United States and, crucially, across the wider world.

Mr Giuliani comes, in several spheres, close to meeting these stern standards. He was an effective if controversial Mayor of New York City, whose deft response to the events of September 11, 2001, revealed an administrative competence and flair for a platform of the highest calibre. He has taken a consistent interest in foreign affairs and many of his instincts there are admirable. He is not tainted with recent Washington politics. He could appeal to electors who are not natural members of the Republican constituency. His nomination would have the virtue of hinting that a willingness to champion a certain sort of narrow social conservatism is no longer a litmus test for aspiring Republican leaders.

Yet there is also a dark side to this intriguing character. His performance at the Justice Department under Ronald Reagan and at times in New York showed a disturbing ruthlessness and a tendency towards cronyism that could easily emerge as an embarrassment, were he to assume the mantle of Washington, Jefferson and Lincoln. His authoritarian streak would not make for a conducive dialogue either with a Congress that is likely to stay in Democratic hands irrespective of the presidential ballot or with those at the helm of important overseas nations. He is a risky option for Republicans.

Compared with three others in the field, this might be considered acceptable. It is a struggle to make a moving argument for Mitt Romney, Fred Thompson and, in particular, Mike Huckabee as a potential president. Mr Romney is a talented businessman and was an impressive Governor of Massachusetts in his one term there, but the Oval Office is in a different league entirely. He has been inconsistent on the key issues over the space of a very short time, and his experience and knowledge of international affairs is modest. He does not truly personify change from the Bush blueprint. Mr Thompson is an amiable soul but seemed bored during his eight years as a senator and is basing his pitch in this battle more on the basis of “none of the above” than on anything more positive. Mr Huckabee, a former Governor of Arkansas, is a “good ol' boy” Southern populist with an economic stance that borders on the incoherent and a foreign policy stand, such as it is, that invites the charge of ignorance. The Democrats would eat him alive if he were selected.

The man that they fear, by contrast, is John McCain. He has long been a politician of consistent, courageous and independent leadership. His positions on domestic politics and America's role in the world are well formed. He would be a different kind of Republican president to the incumbent. He is the one possibility who embodies change and is emphatically of the quality required for this most demanding of political positions. The only half-decent retort that can be made against him is that at 72 he would be the oldest man to start on a first term as president. Perhaps politics needs an older figure to understand and then to overcome the hurdle that history offers the Republicans in 2008.