CHICAGO - With the race for the Republican presidential nomination at its most critical moment to date, only days before the crucial New Hampshire primary, the six candidates will square off Saturday night in Manchester at an ABC News/Yahoo! News/WMUR debate that has the potential to be the most explosive one yet.

The debate, the only broadcast network debate in prime-time before the primary, will air from 9-11 p.m. ET from Saint Anselm College, where it will be moderated by ABC News' Diane Sawyer and George Stephanopoulos, as well as WMUR-TV anchor Josh McElveen.

For every candidate, there is a lot at stake.

Fresh off his narrow eight-vote victory in the Iowa caucuses, Mitt Romney is the favorite to win the nation's first primary on Tuesday. A resounding win in New Hampshire would leave Romney poised to run away with his party's nomination with the primary season only just begun. The former Massachusetts governor has held massive leads in New Hampshire polls throughout the past six months, and he also sits atop the polls in South Carolina, where the race heads next. Convincing wins in both states would make Romney almost unstoppable before the race has even hit Florida in late January.

That only increases the odds that Romney will face a slew of attacks from his rivals at the debate.

Rick Santorum, riding a wave of momentum after his virtual tie with Romney in Iowa, comes into the debate with much to prove. The former Pennsylvania senator was almost an afterthought for much of last year, buried in the polls, but he surged to the forefront in Iowa.

Now the question is whether or not Santorum can survive in the spotlight and avoid the same fates that have befallen the other candidates who have attempted to emerge as the chief alternative to Romney. Just ask Rick Perry, Herman Cain, and Newt Gingrich. All three enjoyed stints atop the polls only to then plunge back into the pack.

With a bare-bones campaign organization and a meager war chest, the little-known Santorum is out to prove that he has staying power. In Iowa, Santorum rode a wave of social conservative support, but those voters are in comparatively short supply in New Hampshire. Expect Santorum at the debate to try to draw clear contrasts with Romney, such as emphasizing his blue-collar roots.

Santorum this week has already struggled to duplicate his Iowa magic in New Hampshire. He gave long-winded answers at numerous campaign stops and on Thursday came in for a frosty reception from a college crowd. At an event with students in Concord, Santorum was even booed after denouncing same-sex marriage, which is legal in New Hampshire.

"[For] 230 years marriage has been between one man and woman. So if you want to change the law … you have to make the positive argument about why," he said, prompting several members of the audience to voice their disagreement with his stance.

If Santorum is eager to go toe to toe with Romney, then Gingrich is positively chomping at the bit. After Romney's political action committee blasted Gingrich, an attack that played a part in Gingrich's recent slump, the former House speaker is now prepared to take off the gloves and duke it out with Romney, a change of strategy after his initial promise to run a positive campaign. However, Gingrich must strike a delicate balance in Manchester if he is to succeed: go on the offensive against Romney without appearing angry and frustrated. In an interview Friday with George Stephanopoulos on "Good Morning America," Gingrich sought to cast himself as a Reagan conservative while dubbing Romney Moderate Mitt, a line that he repeated later in the day at a campaign stop in the Granite State.

"There's a point when you just have to say to somebody - get real. He's a Massachusetts moderate," Gingrich said in Newport, N.H.

Ron Paul, who came in third in Iowa, has proven that he can get under the skin of Gingrich and Santorum. The Texas congressman is not one to shy away from debate duels. At the last Republican debate in Sioux City, Iowa, he went back and forth with Michele Bachmann in a heated exchange. Now Bachmann is out of the race, leaving Paul free to set his sights on other rivals. With consistent support in most New Hampshire polls, Paul may also feel emboldened to deliver an aggressive performance in Manchester.

While there is a lot on the line for Paul, Gingrich, Santorum and Romney, perhaps no candidates need a good debate more than Rick Perry and Jon Huntsman. The former left Iowa with his campaign in tatters after a disappointing fifth-place finish. Perry said he wanted to return to Austin, Texas, to take time to reassess his struggling campaign, leading many observers to expect him to drop out altogether. But a mere 12 hours later, Perry had settled on soldiering on in the GOP contest, with his sights set firmly on a good finish in South Carolina. Despite his focus on the Palmetto State, Perry desperately needs to put in a strong showing in Manchester to jumpstart his campaign.

Huntsman comes into the debate with the same goal but with a completely different focus: He has staked his entire campaign on New Hampshire. Huntsman's decision not to make a push for Iowa or South Carolina leaves him in dire need of a victory in New Hampshire, something that looks unlikely with his weak poll numbers, despite an endorsement by the Boston Globe. His tenuous position, combined with his tendency to take on Romney at every turn, makes a fiery performance from Huntsman in Manchester nearly a guarantee.

With so many candidates in such desperate positions, it is a safe bet to expect fireworks at the debate, especially involving Romney - the frontrunner is likely to bear the brunt of the attacks in Manchester. How successful the attacks are - and how well Romney responds to them - could go a long way toward determining the fate of the GOP race.

Matthew Jaffe is covering the 2012 campaign for ABC News and Univision.