LOS ANGELES (REUTERS) - Former Republican presidential candidate Mitt Romney may be a very wealthy man but when a filmmaker decided to document his two failed White House bids over six years, he was surprised to find how thrifty the former private equity executive was.

"I was surprised that somebody that rich would be that cheap," director Greg Whiteley told Reuters, regaling a story of how Romney was shocked at the price of a glass of milk at a hotel. "He was constantly agonising over how much for a (campaign advertising) spot."

Mr Romney, who left the private equity firm Bain Capital he co-founded to enter politics in 2002, has an estimated net worth of between US$190 million (S$242.1 million) and $250 million.

"He would look at the hotel bill and just go crazy, and say,'I'll just go buy my own milk at the grocery store at a third of the cost'," the director said with a laugh.

The story of Mr Romney's thriftiness is part of Whiteley's new documentary that goes behind the scenes during Mr Romney's first unsuccessful attempt to become the Republican presidential nominee in 2008 and his 2012 campaign for the White House.

Mitt, premiering in Salt Lake City on Friday as part of the annual Sundance Film Festival in Utah, kicks off Romney's story in 2006, in a cozy scene on Christmas Eve around a log fire, when he takes a family vote on whether he should run.

While the film offers a glimpse into intimate moments of the Romney family, there are also times when Mr Romney's wife Ann, or his five sons, express anger and frustration at comments and situations critical of the family patriarch.

In one scene, Mr Romney's son Josh speaks openly to the director, giving a candid translation of his trained answers to the media.

"This is why we don't get good people running for president. What better guy is there than my dad? ... And yet, we keep getting beat up," a tired Josh Romney says.

"They're clean-cut people who are polite and gracious, but I think it's impossible to go through that without developing a degree of cynicism. I think it's a tribute to them that they remained pretty bright-eyed," Whiteley said.

CAPITALISING ON SUNDANCE WITH NETFLIX

Whiteley, who said he had no political affiliation with Mr Romney, made a deal with the former Massachusetts governor that the film would not be released until after the campaign, or if Mr Romney was successful, after his presidential term was over. In return, he had complete editorial control.

Whiteley has also partnered with online video and streaming service Netflix, which is becoming a strong distribution medium for independent films and documentaries.

The filmmaker, who previously showcased his 2005 film New York Doll at Sundance, said it has been difficult to benefit from positive reviews at festivals because of the subsequent wait time for wider theatrical release.

With Netflix, Mitt will be released internationally on Jan 24, and will be better able to capitalise on publicity.

"This way you're able to ride piggy-back on all of the great PR and buzz and momentum from Sundance instead of losing it," Whiteley said.

After convincing Mr Romney to let him film with the help of Mr Romney's wife and son Tagg and with complete editorial control under his belt, Whiteley said he wanted to explore the stress that a campaign for the White House would take on any candidate's family, regardless of which side they are on.

"When you're making a movie about a particular presidential candidate, you run the risk of alienating at least half of your potential audience," Whiteley said.

"I really felt very strongly that this movie was not about a political agenda, but this movie was about a family and the father of this family who is running for president," he added.

The film show the campaign's ups and downs, from high moments such as his successful first debate against President Barack Obama in Denver, to low points like when he was heard on camera criticising the 47 per cent of Americans on government benefits.

Early in the film, Mitt is asked by a family member, "How do you write a concession speech?" in a foreshadowing of what is to come, and the same question is echoed again in 2012 on election night.

For Whiteley, one thing he said he noticed was how much the entire campaign process came down to "a combination of campaign resources and luck".

"There were so many instances in which things could just turn on a dime so quickly. You have a relatively small portion of the electorate determining a party's candidate, and how often in a moment, things would change."