Cain ended his presidential bid in December after sexual harassment allegations [Reuters]

Former Republican presidential hopeful Herman Cain has endorsed Newt Gingrich's bid for the White House, just days ahead of the crucial Florida primary.

Cain, who ended his presidential campaign in December under the weight of sexual harassment allegations, made the endorsement at an event with Gingrich in West Palm Beach, Florida.

It is unclear how much impact it will have for Gingrich in Florida, which has been trending towards rival Mitt Romney ahead of the state's vote on Tuesday.

"I hereby officially and enthusiastically endorse Newt Gingrich for president of the United States," Cain said on Saturday.

Romney, already confident after strong performances in two Florida debates, is gaining ground over Gingrich for the nomination to take on Democratic President Barack Obama in the November 6 election.

Just three days ahead of the pivotal primary race that could determine who has the momentum to win the Republican state-by-state nominating battle, Gingrich and Romney spent the day travelling around Florida in a search for undecided voters.

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A Reuters/Ipsos online poll released on Saturday showed Romney, a former governor of Massachusetts and off-and-on Republican front-runner, with an 11 percentage point lead over Gingrich, up from 8 points a day earlier.

Romney drew support of 43 per cent of likely voters in Florida's January 31 primary and Gingrich 32 per cent.

Gingrich, stung by what he considers unfair attacks from Romney, told reporters in Port St. Lucie that no matter the outcome on Tuesday in Florida, he will keep battling until Republicans formally nominate their candidate at a convention in Tampa in late August.

"I will go all the way to the convention. I expect to win the nomination," he said.

Romney needs a victory on Tuesday to regain his footing after losing badly to Gingrich in the South Carolina primary
last weekend, the third nominating contest.

Gingrich, a former speaker of the US House of Representatives, needs a Florida win to solidify the front-runner mantle he took on after his South Carolina victory.

Source: Agencies