The battle for the Republican presidential nomination always moves first through Iowa, New Hampshire, South Carolina and lately Nevada. But before the 2012 candidates ever get to those states, they have been forced to run through an unexpectedly significant proving ground: nationally televised debates.

The GOP contenders will meet here Tuesday night for their fifth exchange since Labor Day and their eighth of the year. Once considered forums that only occasionally had a real effect on a nomination battle, the debates this year have been the defining feature of the contest.

“For the first time in decades, primary debates aren’t a sideshow, as candidates are using them as a major platform in lieu of early TV ads to project their ideas, personalities and candidacies,” said Jonathan Collegio, communications director for the GOP group American Crossroads. “Campaigns have been smart to figure out that with all the increasing news coverage, a few strong debate performances are worth more than millions on early TV ads, and a weak appearance is worth more than a book” of opposition research.

Debates have played a bigger role in part because the competition in the early states has been so splintered. Four years ago, for example, candidates were organizing extensively in Iowa and New Hampshire, and spending money on ads there. This year, there is nothing comparable in those states. In Iowa, it’s not even clear how many contenders will compete.

To some strategists, debates — which, after all, are only one aspect of campaigns and only one measure of the skills a president needs — have hijacked the process.

“The problem is that the primary election has become almost solely about debates, which has completely overwhelmed the campaigns in a never-ending avalanche of logistics related to requests, scheduling, qualifying and preparation,” said Mark McKinnon, who was a media adviser to President George W. Bush.

The most obvious evidence of the debates’ impact is the change in fortunes for Texas Gov. Rick Perry. He surged to the top of opinion polls after joining the race in August and has watched his support erode after several mediocre debate performances. Were it not for the forums, Perry might still be neck and neck with former Massachusetts governor Mitt Romney. Instead, he is in third place.

In contrast, Romney, through a series of strong performances, has used the forums to solidify position and, more important, to try to convince the sizable number of Republicans who are still skeptical of him that he would be the party’s strongest candidate against President Obama.

“The debates have kind of been a testing ground for a bunch of people not well known by the potential voters,” said Ed Rollins, who was Rep. Michele Bachmann’s campaign manager earlier this year. “There is a shifting group of tea party/conservative voters who are reacting to ‘I like him/I don’t’ rationale, and they are still making up their minds who they are going to vote for.”

In past campaigns, single debates have sometimes been significant. In 1984, Democrat Walter Mondale helped turn back a stiff challenge from Gary Hart with his famous “Where’s the beef?” comment.

In the 2008 contest, Hillary Rodham Clinton saw her high-flying candidacy start to unravel after a misstep on immigration during a debate in Philadelphia. Three months later, Obama may have hurt his chances of winning in New Hampshire when he appeared to rudely dismiss Clinton in a debate just days before the primary.

But taken as a whole, many GOP strategists say, this year’s debates have had a far greater effect. That’s in part because of the volatility of a considerable portion of the Republican electorate.

“The debates have been much more consequential this year than in the past for two reasons,” said Republican pollster Whit Ayres. “The only well-known candidate — Romney — has significant pockets of resistance in the party. And the alternatives to Romney, other than [former House speaker Newt] Gingrich ■ are all relatively unknown nationally.”

Not everyone agrees that the debates have greatly influenced the GOP race. They argue that the contest is so fluid that, by the time the primaries and caucuses begin, the debates may be remembered more for shaping the commentary than for influencing the ultimate outcome.

Mike Murphy, a longtime GOP strategist, remains skeptical. “They’ve been huge in the echo chamber and have made the early (and overrated) national polling gyrate, but meaningful impact in Iowa, New Hampshire and South Carolina is not yet known.”

Jim Dyke, a GOP strategist in South Carolina, also isn’t convinced.

“They are informative and can identify possible themes, candidate character traits and issue depth, but are typically not determinative,” he said. “Mostly, they develop perceptions that campaigns will work to reinforce or dismiss, and it’s how or whether campaigns address issues that can be most informative.”

Perry isn’t the only candidate whose fortunes have been affected by the debates. Former Minnesota governor Tim Pawlenty’s candidacy never recovered after he missed an opportunity to challenge Romney at a forum in New Hampshire. That same night, Bachmann (Minn.) used her first debate to light up the stage and boost her standing. She used a second debate a month later to propel her to a victory in the Iowa straw poll.

Businessman Herman Cain has seized on the opportunities provided by the exchanges to raise his profile, projecting an upbeat personality with a tax plan whose “9-9-9” simplicity has won him early support and produced the latest surge in the polls.

Gingrich (Ga.), whose campaign started with a series of major missteps, has used the exchanges to repair some of the damage. Rep. Ron Paul (Tex.) has used them to draw contrasts with others, particularly on foreign policy. Former senator Rick Santorum (Pa.) and former Utah governor Jon Huntsman Jr., generally relegated to the far reaches of the stage, nonetheless have tried to find weaknesses in their rivals in the hope of moving up.

The competition to stage debates is almost as intense as the contest among the candidates. Tuesday’s forum will be held by CNN, and it will be the cable network’s third debate. Fox News also has held three exchanges. NBC News and Politico co-sponsored a debate at the Ronald Reagan Presidential Library in Simi Valley, Calif. Last week’s New Hampshire forum was held by The Washington Post and Bloomberg News. [For the record, I was part of the team that helped plan it.]

With their often-glitzy sets, vocal audiences and introductions that sometimes sound straight from the sporting world, the debates are infotainment for the politically interested. They have become a form of reality TV, competitions familiar to many Americans. That may explain why they have been such big draws. The Pew Research Center reported Monday that 36 percent of Republicans said they had watched at least one debate.

“Americans are used to watching competitions involving large numbers of contestants and watching the fields get whittled down,” said Tom Rath, a GOP strategist and Romney adviser based in New Hampshire. “The debates have made the campaign accessible to the public in an understandable format much earlier and easier than ever before.”