WASHINGTON — Opinion polls Wednesday showed a surge in support for Republican presidential hopeful Newt Gingrich as he vies to upset frontrunner Mitt Romney in South Carolina and turn the contest on its head.

The polls -- one a national survey, the other just in South Carolina -- suggested the former House speaker had closed the gap substantially on Romney after a bravura performance in the last Republican debate on Monday.

Gingrich's chances rest heavily on how he fares on Saturday in South Carolina, which neighbors his native Georgia and where he has been playing himself up as the true conservative, while constantly jabbing at Romney.

A CNN/Time/ORC survey found Romney leading Gingrich 33-23 percent among likely South Carolina voters, just half of his 19-point lead two weeks ago. The poll had an error margin of plus or minus 4.5 percentage points.

Gingrich, 68, has worked to position himself as the strongest conservative challenger to Romney, who faces stubborn doubts about his conservative credentials and has yet to rally a majority of Republicans behind him.

A Rasmussen national poll had Romney, the former governor of Massachusetts, practically unchanged on 30 percent, Gingrich surging on 27 percent and conservative former senator Rick Santorum shedding support fast on 15 percent.

However, the poll also showed 70 percent of Republican voters nationally believe Romney will ultimately win the nomination to take on President Barack Obama in November. Just 13 percent thought Gingrich would get the nod.

The flash Rasmussen poll was conducted on Tuesday night, after Gingrich's strong debate performance on Monday in Myrtle Beach, South Carolina. The CNN/Time/ORC poll was conducted Friday through Tuesday.

Romney, 64, won the first two contests in the state-by-state process and will become the overwhelming favorite for the nomination if he can clinch a victory in South Carolina on Saturday.