Editor's note: Tune in Thursday at 8 p.m. ET for the CNN-Southern Republican Presidential Debate hosted by John King and follow it on Twitter at #CNNDebate. For real-time coverage of the South Carolina primary, go to CNNPolitics.com or to the CNN apps or CNN mobile web site.

Charleston, South Carolina (CNN) -- The four remaining major Republican candidates are debating for the final time before Saturday's pivotal South Carolina primary, capping a day that shook up the race like no other so far.

Former Massachusetts Gov. Mitt Romney has been the front-runner in the Palmetto State but four polls released over the last two days show his lead nearly evaporated.

Those same surveys show former House Speaker Newt Gingrich surging and basically knotted up with Romney for the top spot. Gingrich on Thursday grabbed a major endorsement as Texas Gov. Rick Perry suspended his bid for the White House and backed Gingrich.

Former Sen. Rick Santorum got a boost on Thursday when the Iowa GOP released the final certified results from its January 3 caucuses, in which Santorum topped Romney by 34 votes. The initial results from the contest that kicked off the presidential primary and caucus calendar had Romney edging out Santorum by eight votes.

And Rep. Ron Paul of Texas had a solid third-place finish in Iowa and a strong second-place finish last week in New Hampshire, but most political pundits saw his performance in Monday's debate in Myrtle Beach, South Carolina, as strained.

With the stage down to four candidates, each man will get plenty of time to talk and to take on his rivals for the nomination. But the spotlight may shine a bit brighter on the two men in the middle, Romney and Gingrich.

Gingrich had rave reviews from his performance in Monday night's showdown, when he repeatedly went after Romney. Will he have an encore performance?

"Gingrich has all the charm and warmth of a chainsaw but right now, Republicans want to cut Washington to ribbons," said Alex Castellanos, a GOP strategist and CNN contributor. "In this debate we'll see how far he's willing to go. Is he willing to cut his own party to shreds to win?"

As for Romney, with his numbers slipping, a strong performance is a must.

"Debates are alpha dog battles, tests of strength and leadership. Romney's passive and defensive performance earlier this week can't be followed by a similar performance. He's going to be tested on the tax issue and needs to show strength, stability and presidential steel," said Castellanos, who was a top media adviser for Romney's 2008 nomination bid but who is not taking sides this cycle.