CHARLESTON - Texas Governor Rick Perry will quit the US presidential race on Thursday and endorse former House speaker Newt Gingrich for the Republican nomination, US media reports said.

Perry's campaign had been flagging for a while but the endorsement represents a major boost for Gingrich as he seeks to overcome Republican frontrunner Mitt Romney in South Carolina's crucial primary on Saturday.

The Perry campaign said in a statement that he would hold a press conference in North Charleston at 11:00 am (1600 GMT).

Washington news website Politico said Perry would make the bombshell announcement there, and The New York Times corroborated the information.

The four remaining candidates -- Gingrich, Romney, Christian conservative Rick Santorum and veteran Texas congressman Ron Paul -- will battle it out on Thursday night in the final South Carolina debate before Saturday's vote.

Romney was originally declared the winner in the first state in the nominating process, Iowa, only to find out Thursday after a final count that Santorum was actually the narrowest of victors.

Romney won convincingly in second-to-vote New Hampshire the following week and has since been seen as the man to beat in the race for the Republican nomination to take on President Barack Obama in November.

Gingrich had trailed Romney by double-digits in South Carolina polls. But the latest surveys, after a bravura debate performance by Gingrich on Monday, showed him closing fast on the former Massachusetts governor and multi-millionaire venture capitalist.

Gingrich 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.

Perry, a tough-talking Texas governor, answered the prayers of core Republicans when he entered the race in mid-August, vaulting to the front of the pack as the ultimate non-Romney candidate.

But the photogenic 61-year-old US Air Force veteran with a thick crop of dark hair came unstuck in the debates, displaying a weak grasp of policy and making a series of embarrassing gaffes.

Perry appeared to put the final nail in his campaign's coffin in mid-November when he forgot a crucial part of his stump speech and stammered for an agonizing 53 seconds.

The states' rights champion named the departments of commerce and education as targets for elimination but could not come up with the third, the energy department.

"The third one, I can't, I'm sorry, I can't. Oops," he concluded.