If they had their way, Republican elders would surely never have chosen Iowa as the starting point of the process to determine the party's presidential nominee. Small and overwhelmingly white, with a Republican electorate heavily dominated by social conservatives, the Midwestern and heavily agricultural state is distinctly unrepresentative of early 21st century America.

Nor, even more certainly, would party bosses have chosen the current selection of contenders to challenge President Obama in November. Many of the potentially strongest candidates – among them Mitch Daniels and Chris Christie, sitting governors of Indiana and New Jersey, respectively, as well as Jeb Bush, the son and brother of former presidents – for differing reasons chose to take a pass on 2012.

Of those actually in the race, both published polls and the private opinions of most Republican strategists suggest that Mitt Romney has much the best chance of defeating Mr Obama in the general election – even though the former governor of Massachusetts has conspicuously failed to excite the party base. In politics, as in life, however, one must go with what one has. Only one in five of the state's registered Republicans may actually bother to participate in them, but tonight, as in every election since 1976, Iowa's caucuses formally kick off the contest to select the most powerful man on the planet. And with them will begin a winnowing of the field.

The first to go may be Michele Bachmann, the conservative Minnesota Congresswoman who has staked all on Iowa, but whose support now languishes in single digits. Barring a small miracle, John Huntsman, the former governor of Utah and US Ambassador to China, will follow her after next week's primary in New Hampshire.

The signs are that the result in Iowa will be close. Mr Romney is in a virtual tie with Ron Paul, the quirky libertarian Congressman from Texas. Rising fast in the polls is the improbable figure of Rick Santorum, the former two-term senator whose ringing Christian conservatism has led Iowans to forget that, in 2006, he was thrown out by Pennsylvania's voters by the largest margin they ever inflicted on an incumbent.

The other two candidates, the erstwhile House Speaker Newt Gingrich and Rick Perry, the governor of Texas, will probably do well enough to stay alive, in the hope of resurrecting their campaigns with strong performances in the remaining primaries in South Carolina and Florida later in the month.

In a contest marked above all by a deafening lack of enthusiasm for Mr Romney, and in which almost all his rivals have surged, only to fall quickly back to earth, any prediction is unwise. There are, however, two most likely scenarios. Should Mr Romney prevail in Iowa and follow up with a big win in New Hampshire, where he has long been far ahead in the polls, he could wrap up the nomination very quickly. But if Iowa's hitherto divided social conservatives rally behind a single candidate who emerges triumphant tonight, then a long and bruising battle is in prospect, and the prime beneficiary will probably be Barack Obama.

Either way, it will soon be clear whether the Republicans are serious about regaining the White House. Unlike 2008, this is an election the party can win. Personally, Mr Obama remains reasonably popular. But, as he struggles with the worst economy in decades, he has inevitably lost that magical aura of four years ago. His good fortune has been the quality of his opponents, above all in Congress, whose dysfunction is rightly blamed by voters on Republican intransigence. Thus far, events on the Republican campaign trail suggest that his luck continues to hold.