Just when you think you know how the game is played, someone always changes the rules. This year’s Republican primary season seems ready to challenge one of the oldest and most reliable rules of Republican primaries, the one that says that the most conservative candidate usually wins.

After I was taught this rule by some wise senior consultants, for years I sought out exceptions in order to prove that it couldn’t be that simple. Surely the factors determining the outcome of a primary would be more complex than mere ideology of the contestants, I reasoned. But after several decades of active skepticism I am convinced that this rule is pretty much immutable, and at least as accurate as the typical TV news weathercast.

The best I have been able to do in refining the rule is add a slight modification of the principle. I now articulate the rule like this: “The most right-leaning legitimate conservative usually wins.” This slight reformulation adds a necessary hedge to the proposition, diminishing the prospects of “illegitimate” candidates.

There are at least two categories of candidates that merit the illegitimate label. Right-wing fringe candidates and nut-jobs, no matter how conservative, don’t get the guaranteed win. This explains how a Pat Buchanan could never succeed in his presidential bids against clearly more moderate opponents. Buchanan went outside the boundaries of legitimate conservative orthodoxy.

The second type of illegitimate contender is one who simply doesn’t raise enough money to have the capacity to adequately tell enough voters his story. Some may feel it’s a sad state of affairs when an otherwise solid conservative candidate is deemed illegitimate simply because he cannot raise enough money to tell his story, but that’s the way it is. The ability to raise money is one crucial element of demonstrating legitimacy. For example, in the current field, Rep. Duncan Hunter (Calif.) is undeniably a solid conservative whose views are mostly in line with conservative orthodoxy. But because he cannot raise sufficient funds to compete, he trails even more oddball candidates like Rep. Ron Paul (Texas).

So how is the rule working this time? Is the most conservative legitimate candidate winning? I don’t think so. Going by everything I know, Fred Thompson is undeniably the most consistent conservative in the field. And while he has shown some progress lately, finishing third in Iowa’s caucuses, his overall prognosis, judging from polls, is not encouraging.

Why isn’t the conservative stalwart performing better? Some would blame Thompson’s reserved style of campaigning, I don’t think this is a valid criticism. Thompson has campaigned at least as hard as some of the Republicans polling ahead of him. I believe that he’s campaigned long and hard enough that if you asked Republican primary voters to say whose views are most conservative of the front-runners — Rudy Giuliani, Mitt Romney, John McCain or Thompson — 70 percent or more would say Thompson. Really, his only competition for the conservative mantle is Romney, and the former Massachusetts governor gets criticized often for flip-flops on conservative litmus-test issues.

There may be idiosyncratic explanations for why the rule isn’t working in the current presidential primary cycle. For example, many conservative Republicans may so admire John McCain’s well-documented service to the nation that they overlook his more moderate positions on a variety of issues. Other conservatives may be so focused on having a president who will get harsh with militant Islamic terrorists that they are persuaded by Giuliani’s tough-guy image.

The more ominous explanation, at least for conservatism, is that party members have changed.

Republicans, as a whole, are not as conservative as they once were. Research results I am seeing suggest to me that this is key to why the rules are changing. Conservatives no longer benefit from the domination they once enjoyed.

Hill is director of Hill Research Consultants, a Texas-based firm that has polled for GOP candidates and causes since 1988.