With former Sen. Fred Thompson’s official declaration of his candidacy, the race for the Republican presidential nomination has now assumed the shape that seems likely to characterize it right down to the finish line. Conceivably some new and unexpected contender could still enter the contest and win it, as Wendell Willkie did in 1940, but the odds against such a development are high. Some observers, noting that the national political conventions won’t be held until the late summer of 2008, argue that there is still time for a surprise. But by advancing the dates of many of the most important primaries to early next year, the parties have dramatically shortened the time actually available. Within five months — by early February — we will almost certainly know the identity of the Republican and Democratic candidates who will be officially nominated next summer. (And that, incidentally, will raise an interesting question for both of them: Just how will they maintain their momentum from early February to Election Day, Nov. 4?)

In the case of the Republicans, the striking fact is that the field of serious contenders is still so large. At this point, it is certainly possible that any one of at least five men — Sen. John McCain of Arizona; Thompson of Tennessee; former Gov. Mitt Romney of Massachusetts; former Gov. Mike Huckabee of Arkansas, and former Mayor Rudy Giuliani of New York City — could easily win the nomination. (There are a few other possibilities as well, witness the recent “debate” among Republican aspirants, but those five are certainly the standouts at the moment.)

The remarkable thing is that not one of these men has established a truly formidable lead. Giuliani has held onto the top spot in most national polls of Republican primary voters, but this amounts in most cases to about 30 percent, with various rivals close behind; it is certainly not enough of a margin to make him a prohibitive favorite. Romney, interestingly, leads the field in Iowa and New Hampshire, which are both key states with primaries early in the year, and victories there (if he achieves them) would give him an important boost. McCain, who was the early frontrunner, has seemed to fade in recent months, but is attracting renewed attention as the generally acknowledged “winner” of the last debate. Moreover, he is the chief hawk among the candidates on the war in Iraq, and any improvements there (not to mention any fresh terrorist attack on the United States) would help his chances. Huckabee is an attractive outsider who has talked his way into semi-serious consideration. Thompson is thought to provide the element of robust conservatism that his rivals supposedly lack, but his hesitation in entering the race, and perhaps some doubts about the intensity of that conservatism, may hamper him.

So it is not only a wide, but widely disparate, field. The voters in the Republican primaries will pick the winner, and it is important to remember that this is a group of people by no means are necessarily representative of the American electorate as a whole. The candidates, therefore, will have to adopt positions in the primaries that may not resonate all that well among the voters at large in November. On Iraq, for example, all five candidates resolutely support the “surge,” and favor pressing on to victory there. That’s catnip to most Republicans likely to vote in the primaries. But, while the Democrats are very probably overestimating the public’s desire for a prompt and disastrous pull-out, polls do show that most Americans have serious doubts about the likelihood of an American “victory.” So the Republican nominee, whoever he is, will probably have to modify his rhetoric on the subject of Iraq between February and November.

So my guess is that the choice of a Republican nominee, in the early primaries of 2008, is likely to be heavily influenced by events between now and then. Further progress in Iraq, modest (as hitherto) or more dramatic, would certainly have an effect. Another successful assault on the American homeland, along the lines of 9/11 or even worse, would probably have an absolutely transforming effect, not only in the primaries but in the general election. Being known as a good man in a fight might turn out to be the most valuable characteristic of all.

(William Rusher is a Distinguished Fellow of the Claremont Institute for the Study of Statesmanship and Political Philosophy.)