With the crucial Republican presidential primary in South Carolina just hours away, long-time front-runner Mitt Romney is acknowledging what some opinion polls are suggesting: He could lose Saturday.

The idea that the former governor of liberal Massachusetts may not win the primary in a state where conservative evangelical Christians make up about 60 percent of Republican voters isn't that surprising.

But Romney's path to a neck-and-neck finish with former House of Representatives Speaker Newt Gingrich has begun to look like a lost opportunity, defined by Romney's reluctance to reveal more about his vast wealth and his repeated inability to explain why.

The former private equity executive's discomfort in discussing such personal matters was again evident in Thursday night's debate in Charleston.

When asked whether he would release 12 years of tax returns as his father, George, had done while running for president in 1968, Romney said through a thin smile, Maybe.

The answer drew a few catcalls from the conservative audience, and contrasted sharply with how Gingrich deftly turned a question about cheating on his second wife into an attack on the media that drew a standing ovation.

It may have been the defining moment of the campaign in South Carolina, the third contest in the state-by-state race to determine who will face Democratic President Barack Obama in the November 6 elections.

As Romney tried to pump up supporters' enthusiasm Friday and launched new attacks on Gingrich, a question hung over Romney's campaign: Why does he have such difficulty answering questions about his money?

His wealth, now an estimated $270 million, has been an issue during his previous runs for office -- notably in 1994, when he lost a U.S. Senate race to incumbent Democrat Ted Kennedy.

Romney said Friday that he has been more focused on campaign issues such as jobs and the economy, and acknowledged that he may not have handled questions about his finances as well as he could have.

I can't possibly tell you that everything I do in the campaign is perfect, he told reporters here.

Voters, pundits and others offered other theories about his problems dealing the issue of his finances.

Like Gingrich, some questioned whether Romney has something to hide.

Perhaps the most intriguing theory came from Michael Kranish and Scott Helman, co-authors of a new book on Romney called The Real Romney.

During an interview on CNN, Kranish said a Romney family member told the authors that Romney's cautious manner while campaigning -- which can make him seem distant and stiff -- partly reflects a lesson he learned from his father.

George Romney was a chairman of American Motors and a Michigan governor. As a presidential candidate in 1968, he supported the Vietnam War but became an opponent of it after visiting Vietnam with several congressmen.

George Romney later said during a TV interview that his earlier position was the product of brainwashing by generals and others who backed the war.

The comment sunk his bid for the Republican nomination, which was won by Richard Nixon.

That one sentence pretty much exploded his presidential ambitions, Kranish said. And Mitt Romney has taken a lesson from that. ... As a result, Mitt is more careful, more scripted in what he says because one sentence could perhaps end (his) campaign as it did his father's.

Kranish added that within his circle of trusted advisers, Romney is very warm.

With the general public, however, it could be a hard connection to make, especially given his great wealth ... and trying to make that connection to the average person.

Kranish added that Romney could use more of the free-flowing nature that his father had.

'A LONG SLOG'

Just a few days ago, Romney appeared to be in position to take a big step toward wrapping up the Republican nomination by sailing to victory in South Carolina, having won in New Hampshire.

But on Friday, amid signs that what had been a 15- to 20-point lead over Gingrich was shrinking rapidly, Romney and his surrogates were casting the race as a months-long marathon.

This is a long slog, said former New Hampshire Governor John Sununu, a Romney supporter. Mitt Romney's strength has been clearly defined as being ready for a long campaign.

On a rainy Friday, Romney also sought to lower expectations for Saturday, and noted that Gingrich is from neighbouring Georgia.

Speaker Gingrich is from a neighbouring state, well-known, popular in the state, so I knew we'd have a long road ahead of us in South Carolina, Romney said. Frankly, to be in a neck-and-neck race at this last moment is kind of exciting.

As disappointing as it would be for Romney's campaign to finish in any place but first in South Carolina, he remains the best positioned of the four Republican candidates remaining in the race.

He has more campaign funds and a stronger organization than Gingrich and the two other Republican contenders, former Pennsylvania senator Rick Santorum and Texas congressman Ron Paul.

Such advantages for Romney could be key in the next primary, in Florida on January 31.

Florida is one of the largest states in the nation with several major media markets. Maintaining a travelling campaign and covering the state with TV ads is more expensive in Florida than most other states.

I'm still hoping and planning to win here in South Carolina, Romney said. I'm sure the speaker feels the same way I do, but we're going to go on for a long race and I think I've got the staying power and a message that I believe connects with people.

Romney has held big leads in many South Carolina polls, but one survey released Friday by Clemson University showed Gingrich leading Romney 32 percent to 26 percent. Another poll, by American Research Group, showed the race in a virtual tie.

HONESTY, CHARACTER

Romney's campaign has been scrambling since Monday, when his finances became a particularly hot issue.

That was the day Romney acknowledged that his income tax rate is 15 percent. That's well below what most wage-earning Americans pay, and suggests that much of Romney's income is capital gains from investments.

On Friday, Romney's campaign tried to take some of the air out of Gingrich's ascent.

In an apparent dig at Gingrich, Romney said the Republican nominee needs to have vision, trust, honesty, character, integrity.

Santorum, who performed well in Thursday's debate with several jabs at Romney and Gingrich, also has tried to cast Gingrich as erratic.

Romney's campaign called for Gingrich to release documents involving an ethics investigation of him when he was speaker in 1997.

Gingrich scoffed at the suggestion, saying that the documents already were public.

Gingrich's standing in recent polls suggest that his efforts to win support among evangelical Christian voters have not been damaged much by questions about his personal life.

On Thursday, however, a former wife, Marianne, said in an ABC interview that Gingrich had sought an open marriage when he was having an affair in the 1990s with Callista Bisek, who later became Gingrich's third wife.

Gingrich's denial of his former wife's allegation was his shining moment during Thursday's debate.

(Additional reporting by Sam Youngman and Colleen Jenkins; Editing by David Lindsey and Doina Chiacu)