As the Republican presidential hopefuls prepare to face off against one another at Thursday night's Fox News debate in Iowa, each candidate faces a daunting challenge - distinguishing themselves from the rest of the pack, enough to convince voters in Iowa and across the country that they're uniquely fit to defeat President Obama in 2012.

Debates are a crucial test for any candidate, one that could even make or break their campaigns at this early stage. How might the best and worst possible nights for each candidate play out?

MITT ROMNEY

BEST: Romney should "relentlessly contrast himself with Obama on the economy and jobs," said Larry Sabato, director of the Center for Politics at the University of Virginia, "only indirectly jabbing the others."

Texas Gov. Rick Perry's unannounced, but expected, candidacy will loom like a shadow over the debate, said Fox News contributor Dick Morris. "Romney's got to answer the questions about Obamacare, about his social conservatism - satisfy conservatives and show that he can be a passionate standard-bearer against Obama," Morris said. "Because if he can't, Perry will."

WORST: "Getting drawn into a brawl with other candidates, especially the undercard contenders," Sabato said. "Doesn't look presidential."

REP. MICHELE BACHMANN, R-MINN.

BEST: The Tea Party favorite and Iowa native has consistently stayed at the top of the polls since announcing her candidacy in June. Sabato said she should embrace her conservativism - "I'm the new mainstream and I can win in November."

WORST: "Any gaffe on history or public policy," Sabato said.

TIM PAWLENTY

BEST: "Roll the dice and confront both Romney and Bachmann," Sabato said, "explaining why you'd be a better president."

WORST: The former governor has been struggling to find footing in the primary season. "Another poor performance, another poor showing in Ames might knock him out," said Morris. Sabato agrees. "Pulling his punches, looking bland and boring, and letting another opportunity pass him by" would be the worst thing he could do tonight.

NEWT GINGRICH

BEST: Gingrich stressed the importance of the Iowa straw poll Thursday, telling Fox News that the outcome was "the start of a national conversation." Sabato said Gingrich should seize the opportunity to connect with a national audience. "Come across as likeable," would be Newt's best bet, he said, "not just knowledgeable."

WORST: "Giving long, harsh answers as the lecturing professor," Sabato said.

REP. RON PAUL, R-TEXAS

BEST: Paul's staunch libertarian views have at times gone against the GOP grain. Sabato said he should "make a consistent argument that will appeal to a large majority of Republicans, not just the Libertarian wing."

But Morris told Fox that Paul, in the wake of weeks of devastating economic news, should embrace the unconventional wisdom that has long isolated him from the beltway mainstream. "He may be the biggest winner from the events of the last week, because it shows that his very unconventional theories that differ sharply from the conventional wisdom in Washington, really might develop some sport," Morris said. "What he needs to do is to capitalize on those developments to show how his policies that differed from everybody else's in Washington, really could make the difference."

WORST: Some of Paul's remarks, however, have come back to haunt him - notably, a previous assertion about legalizing certain drug use. Paul found himself in hot water after using a comparison in which he stated that even if heroin became legal, it wouldn't mean that people would start using it. The worst thing Paul could do in the debate, said Sabato, is "mentioning anything that evokes the same response as 'legalizing heroin' got."

HERMAN CAIN

BEST: "What Cain's got to do is to really distinguish himself," said Morris. "He's got to make clear how his performance as a businessman, his perspective, differs from the conventional wisdom in Washington." Sabato adds that Cain must "explain convincingly why his lack of office experience will help and not hurt running against Obama."

WORST: "Vague answers to important policy questions that underline his lack of policy depth," said Sabato.

RICK SANTORUM

BEST: The conservative candidate has lingered in the shadows of higher-profile candidates like Bachmann. In his best night, Santorum would "excite the Christian base in Iowa by going hardcore right on abortion and gay rights," said Sabato. "It's his only shot to surge."

WORST: "Canned answers that don't distinguish him from the pack at this late date," said Sabato.

JON HUNTSMAN

BEST: One of Huntsman's biggest challenges is distancing himself from his former boss - President Obama. He needs to "convince Republicans that, despite what they've read and heard, he's a strong conservative," said Sabato.

WORST: "Seeming as mushy and bland as he was at his announcement, and as moderate as parts of his record would indicate he is," Sabato said.