The recent departure of Kansas Senator Sam Brownback from the contest for the Republican presidential nomination highlights an interesting feature of that contest. The Republican race should be seen as a multiple-round elimination tournament that started with three brackets, each of which has pitted two contenders vying with each other for the right to be the one Republican who can plausibly represent a particular niche in the field. (This analysis excludes peripheral candidates like Duncan Hunter and Tom Tancredo. It also excludes Ron Paul, who is in a category of his own.)

The three niches are 1) A tough, maverick Republican who makes conventional conservatives nervous but has potentially strong crossover appeal and is a clear "national security" candidate; 2) A solid, conventional conservative who can appeal reliably to both social and economic conservatives; 3) A long-shot social conservative who offers at least a shadow of "compassionate conservatism," including a de-emphasis on limited government, an openly religious frame of reference, and (within the context of the field) a relatively liberal immigration stand. The notion here is that each candidate has a natural constituency, and to maximize his chance for ultimate success he must avoid splitting his natural constituency with another similarly-positioned candidate. Thus, each candidate’s first task is to win his bracket and knock out of serious contention, if not out of the race altogether, his closest rival.

Accordingly, the three first-round matchups have been Rudy Giuliani vs. John McCain in the first bracket, Mitt Romney vs. Fred Thompson in the second bracket, and Mike Huckabee vs. Sam Brownback in the third bracket. With Brownback’s exit, Huckabee became the first Republican candidate to clear out his bracket and move to the next level. This is undoubtedly in part because his rival was appreciably weaker than the contenders in the other brackets; Brownback was easier to clear out. Huckabee’s success may also be due to his strengths as a candidate, which are only now beginning to receive notice. For whatever reason, that portion of the Republican playoff is now completed.

The other two playoffs are more difficult, and it is hard to predict how they will turn out.

It has long been understood that McCain and Giuliani were angling for a similar constituency, had a similar appeal, and would each gain most likely at the other’s expense. Through the end of 2006, it was widely assumed that McCain had the advantage over Giuliani. For most of 2007, it has been the reverse. Polls continue to show that if McCain were to leave the race, Giuliani would be the biggest beneficiary.

Complicating this contest, however, are two factors. First, each candidate seems to like the other well enough not to want to engage in the sort of struggle that might be necessary to bring a decisive win against the other. As a result, the competition in this bracket has thus far been muted. Second, McCain has the option, which he could exercise at any time, of trying to force his way out of this bracket into another bracket, most likely the solid conservative bracket. Such a maneuver would be difficult, given McCain’s self-conscious maverick campaign of 2000 and his penchant for sticking an occasional finger in the eye of conservatives. Nevertheless, it is not impossible, since McCain’s voting record has always been generally conservative, and he is significantly more conservative than Giuliani on social issues. If Giuliani continues to lead McCain decisively, or even opens up a bigger lead, he might win the bracket by squeezing McCain onto a different field of play.

The solid conservative bracket is not so simple, either, with or without McCain. Thompson has yet to catch fire, and many Republicans are now wondering if he ever will. Romney has high negative ratings and is still struggling to convince Republicans that he actually belongs in the solid conservative bracket, given his famously liberal 1994 campaign for Edward Kennedy’s Senate seat. Thompson does better than Romney in national polls, while Romney is stronger than Thompson in the early states that will probably shape the race. It is conceivable that neither will knock the other out until some distance into the primary season; indeed, such an outcome is probable. It is difficult at this moment to imagine the mechanics of how such a knockout would occur between now and January 3, when Iowa launches the nominating process. The longer it takes for one of them to establish dominance, though, the harder it will be for either of them to win the nomination, especially if Giuliani consolidates his hold on his niche. If McCain gives up on his natural bracket and tries to join them, a three-way split would ensue, delaying even longer the sorting-out process.

The Thompson/Romney bracket represents the traditional center of gravity of the Republican Party, and will be most bitterly fought over. However, the national security/maverick bracket has led the Republican field since there was a field, and is the most competitive against Democratic contenders in head-to-head polls. Huckabee could turn out to be not only the first but the only contender to gain a lock on his niche before the voting starts. He is still a long-shot, but his success raises an interesting question. If Giuliani wins his bracket and Huckabee has won his, will Republicans who form the natural constituency of the remaining bracket become frustrated at the continued deadlock among their favorites and begin looking outside their bracket for the next best thing? And if they do, who will they consider to be the next best thing?

Andrew E. Busch is a Professor of Government at Claremont McKenna College and an Adjunct Fellow of the Ashbrook Center.