By Alistair Bell

COLUMBIA, South Carolina (Reuters) - Republican presidential front-runner Mitt Romney must overcome wily political veteran Newt Gingrich in the South Carolina primary on Saturday to keep his march toward the party's nomination on track.

Gingrich's sudden rise in recent days has presented Romney with the biggest challenge yet in months of campaigning to become the Republican who will face President Barack Obama in November.

With two other candidates trailing in the polls, the primary looks like a straight fight between the two very different men.

A multimillionaire ex-businessman who runs a sleek campaign, Romney has consistently won the support of a quarter of Republicans nationally with his message on jobs and the economy. But he has failed to capture the hearts of many conservatives.

Gingrich is a former history teacher who roams off message and has a checkered past but a killer turn of phrase in debates.

"It's about which one of them can beat Obama," said Vaughan Mureaux, a retired school administrator in South Carolina who originally supported Romney but was impressed when he attended a Gingrich speech this week.

Fueled by a grudge that has become almost personal, former House of Representatives Speaker Gingrich has sown seeds of doubt among Republicans who were beginning to see Romney as the inevitable nominee after strong showings in the first two votes in Iowa and New Hampshire.

Romney has stumbled, acknowledging in the last week he pays a much lower tax rate than most Americans and struggling to answer questions about a planned release of tax records.

With only hours left before the voting in South Carolina, the former Massachusetts governor's campaign tried to turn the tables and ask for more information about ethics violations for which Gingrich was sanctioned in Congress in the 1990s.

"Don't you love these guys? He doesn't release anything, he doesn't answer anything. And he's even confused about whether or not he will ever release anything. And then he's decided to pick a fight over releasing stuff," Gingrich said.

NO LOVE LOST

Animosity between the two has been festering since December, when a group supporting Romney launched a blitz of negative TV ads in Iowa that effectively ruined Gingrich's campaign there.

He has hit back by attacking Romney's business record.

The fight has been bruising in South Carolina, a conservative state with a history of dirty politics.

The pair could not even agree to avoid each other on election day. Both Romney and Gingrich have campaign events scheduled at the same time on Saturday morning at Tommy's Country Ham House in Greenville.

The two campaigns refused to back off on Friday night and change arrangements.

Romney's team is playing up his family values as a father of five boys. That is in contrast to three times-married Gingrich, who has had admitted to infidelities, including with his current wife Callista when she worked in Congress.

Romney's former wife of 42 years, Ann, appears in an ad extolling the virtues needed in a strong president.

"If you really want to know how a person will operate, look at how they have lived their life. And I think that's why it's so important to understand the character of a person," she says.

The winner of South Carolina's Republican presidential primary has gone on to win the party's nomination in every election since 1980. Romney's path to the nomination would be nearly clear if he can clinch the state on Saturday.

He may be helped if the South Carolina conservative vote is split between Gingrich, former Pennsylvania senator Rick Santorum and libertarian congressman Ron Paul.

Romney has been helped in South Carolina by Governor Nikki Haley, formerly a favorite of Tea Party conservatives. "The coolest thing we could ever see is a jobs candidate go up against a government-loving President Obama," she said in Charleston with Romney.

"Get out there and vote," Romney told a crowd of about 200 people. Voting starts at 7 a.m. EST (1200 GMT) and closes 12 hours later.

(Additional reporting by Steve Holland and Colleen Jenkins; editing by Todd Eastham)