Likability has always been important to voters in picking a president.

Republican presidential candidates Rep. Ron Paul, former Massachusetts governor Mitt Romney and former House speaker Newt Gingrich, watch as former Pennsylvania senator Rick Santorum speaks during a Republican presidential debate Feb. 22, 2012, in Mesa, Ariz. (Jae C. Hong/Associated Press)

George W. Bush definitely had more of it than John Kerry and Al Gore. Bill Clinton had it over George H. W. Bush, who kinda had it over Michael Dukakis. Ronald Reagan had it over both Walter Mondale and Jimmy Carter. And Richard Nixon? Well, there's an anomaly.

In fact, likability is just as important as credibility, says Pam Holloway, the marketing consultant who co-authored “Axis of Influence: How Credibility & Likeability Intersect to Drive Success.” In the 2008 elections, for instance, Hillary Clinton came off as cold, while Sarah Palin appeared better able to relate to regular folks.

So how about the likability of the current Republican field of four?

“On the GOP side, it's really, really tough,” Holloway says. “They don't score particularly well in that category.

“We like people who are like us, we like people we can relate to,” she says. “None of them are really relatable.”

For instance, front-runner Mitt Romney appears tone-deaf about his wealth, while Rick Santorum often comes off as sanctimonious.

Newt Gingrich's baby face promotes likability, until he opens his mouth, and his ego flows out, Holloway says. Gingrich has other issues, too.

“I'd say he's pretty credible, but he’d be more credible if he were more consistent,” she says. “One day it’s the moon, the next day it’s drilling in South Dakota.”

Ron Paul has the opposite problem.

“Ron Paul probably scores higher in likability among that group,” Holloway says, but ”I think Ron Paul has kind of a weird face. I think subconsciously people back away from that.”

Meanwhile, President Barack Obama has taken to singing at public events recently, a factor that improves his likability. And he’s known for holding a beer summit to mend fences between a Harvard University professor and a police officer.

George W. Bush also had that human touch, with polls often indicating Americans would prefer to have a beer with him that his opponents — even though Bush doesn’t drink.

“It doesn’t matter what your feelings are in terms of politics, everyone would admit that he’s a likeble guy,” Holloway says of Bush. ”We don't really have anybody in that bunch who you would want to go have a beer with.”

Holloway, in an interview and on her blog, also looks at the GOP race in terms of that Southern saying about telling the unvarnished truth — “how the cow ate the cabbage.”

“Newt would tell you it’s not a cabbage, Santorum would tell you the cow’s not allowed to eat the cabbage,” she says. “Mitt would say ‘I know the owner of the cabbage farm,’ and Ron Paul would say we shouldn’t get involved in whether cows eat cabbages.”

Can Romney learn to be more likeable? Possibly, Holloway says.

“There is what I perceive a sincere effort to get there with Mitt Romney, he just puts his foot in his mouth,” she says. “He's been in this elitist place his whole life, and he can’t be an everyman. If he could, I think it would make a huge difference.”