Normally, the first debate of the presidential primary season serves as a starting gun. The one that will take place on Thursday night could sound more like a distress call.

Consider the contrast with this very week four years ago, when a field of 10 Republican contenders lined up for the first time, onstage at the Ronald Reagan Presidential Library in Simi Valley, Calif. It included four former governors, two sitting senators, three members of the House and a former New York City mayor who had become something of a national hero for his leadership in the wake of the Sept. 11 attacks.

The group that assembles for the Fox News-South Carolina GOP debate in Greenville, S.C., on Thursday will be half the size — and distinguished more by who isn’t there than by who is. Of the presumed top-tier candidates and most-buzzed-about names, only former Minnesota governor Tim Pawlenty has agreed to show up.

That fact alone speaks to the turbulence and uncertainty of a political party known for the orderly way in which it usually picks its nominee, with its all-but-predetermined outcome.

With the strongest possible contenders holding back — not just from debating, but also from gearing up their campaign operations — a queasiness is setting in among Republicans about whether their field will be strong enough to produce a standard-bearer who can beat an incumbent.

It may not seem all that late, given that the first contest, in Iowa, is still nine months away. But in Gallup polling since 1952, the GOP race has almost always had a clear front-runner with a double-digit lead by this point — the exception being Gerald R. Ford and Ronald Reagan’s rematch in 1980. And in nearly every case, that front-runner ultimately received the nomination.

The 2012 primary race, however, shows little sign of jelling. When a Washington Post-ABC News poll asked Republicans and GOP-leaners in mid-April to name whom they would vote for in a primary or a caucus, the leading contender, former Massachusetts governor Mitt Romney, registered only 16 percent.

Other recent polls have shown the race in a fluid three-way tie that includes, in varying combinations, Romney, former Arkansas governor Mike Huckabee, former Alaska governor Sarah Palin, and reality television star and real estate developer Donald Trump.

None of them will be in South Carolina on Thursday night; nor will former House speaker Newt Gingrich (Ga.), Indiana Gov. Mitch Daniels or former Utah governor Jon Huntsman, three others mentioned as possible candidates. Indeed, of that group, only Romney has gone so far as to form an exploratory committee.

In addition to Pawlenty, those slated to participate in the debate on Thursday are former senator Rick Santorum (Pa.); ex-Godfather’s Pizza chief executive Herman Cain; and two libertarians, Rep. Ron Paul (Tex.) and former New Mexico governor Gary Johnson.

So why is the 2012 race getting off to such a slow start?

For one thing, those who went through the process the last time came away convinced that it began too early and lasted too long.

In a recent interview, Huckabee — who says he may not make up his mind until this summer — recalled what became a marathon of debates as being “the same old stuff every time,” driven mostly by the day’s media narrative.

“Of the hour-and-a-half debate, an hour and five minutes of it would be on Iraq,” an issue upon which the candidates were in near-total agreement, Huckabee said. “I can’t imagine most Americans were just sitting there on the edge of their seats, hearing yet another response to Iraq. It turned out that Iraq wasn’t even the driving issue of the campaign. Of the 11 debates, there was not one question on education — not one.”

The traditional calculations have also been upset by some forces within the Republican Party. Where the GOP establishment usually selects a candidate and closes ranks behind him early, many have doubts about presumed front-runner Romney’s political abilities, given his unsteady performance in the 2008 campaign. That’s why they are keeping a light on in the window for someone else — Daniels is their current hope — to enter the race.

In Washington on Wednesday to give a speech on education, Daniels, who wrapped up his final legislative session on April 29, said it has been a “happy surprise” that he still has time to make up his mind about whether to run for president. He said he expects to ponder it for at least a few more weeks.

“From the vantage point of the public, a blessing,” he added. “Unless you’re a political professional or running a bed-and-breakfast in New Hampshire, it’s a darn good thing that we’ll have a nomination campaign measured in months and not in years.”

Meanwhile, the insurgent force of the tea party could give at least some temporary running room to such otherwise-implausible candidates as Trump, Palin and Rep. Michele Bachmann (Minn.).

Then there is the fallout from November’s midterm elections, which put House Republicans, rather than would-be presidential contenders, at the center of political gravity in the party.

House Speaker John A. Boehner (R-Ohio) has “sucked all the oxygen out until now. It has been ‘The John Boehner Show’ — as it should have been,” said political consultant Scott Reed, who had been assisting Mississippi Gov. Haley Barbour prepare for a run until Barbour decided against it last month.

But Reed and others note that crunch time is coming. In Iowa, for instance, activists say that possible candidates must begin organizing by early summer if they hope to do well in that state’s August straw poll.

If the Republicans are looking for reassurance to ease their anxiety about the race’s chaotic start, they need look no further than . . . the Democrats.

The same Gallup polling of the past half a century shows that the party’s voters almost never settle on a front-runner early, which is why such fresh faces as Barack Obama and Bill Clinton have been able to break through and win.

Clinton, then the governor of Arkansas, didn’t even announce his bid for president until October 1991. That year, many in the Democratic Party were in despair about the fact that their better-known figures, among them New York Gov. Mario Cuomo, had decided not to run against a seemingly strong incumbent, George H.W. Bush.

Conventional wisdom, as so often is the case in politics, turned out to be a bad bet.