Republicans jockeying for position to challenge President Obama next year will have their first showdown Thursday night in South Carolina, a state that has held the first presidential primary in the South since 1980 and has been a crucial steppingstone for GOP candidates.

Former Minnesota Gov. Tim Pawlenty, former Pennsylvania Sen. Rick Santorum, Texas Rep. Ron Paul, businessman Herman Cain and former New Mexico Gov. Gary Johnson will face off in Greenville, S.C., in the Fox News-sponsored debate that begins at 9 p.m.

"Whoever has won the Republican primary in South Carolina has gone on to get the nomination," said Scot Huffmon, a political science professor at Winthrop University...