In the Republicans' Las Vegas show this week, the headliner turned out to be Mitt Romney. That was a surprise. Many pundits assumed it would be businessman Herman Cain, a wild card who has recently led the field in polls of Republican voters. While media elites and politicians mock Cain's 9-9-9 tax reform plan, everyday voters give him credit for at least having a plan -- and extra credit for the fact that it is easy to understand.

By contrast, Romney's economic plan has 59 points. Should Americans really need to hire a team of accountants to understand a candidate's sales pitch?

But Romney's major challenge isn't his communication strategy. Instead, it's the same thing that plagued the former Massachusetts governor in the 2008 election: Many Republican voters don't like him, trust him or relate to him. It's not, as some political observers suggest, that they don't think Romney is sufficiently conservative. Rather, it's that they don't think he's sufficiently anything. What does it matter if they agree with what he espouses about issue X or topic Y if they don't believe a word he says?

In the CNN debate in Las Vegas, his opponents tried to draw attention to Romney's character by accusing him -- one after another -- of being inconsistent, dishonest and untrustworthy.

-- Cain accused Romney of deceptively "mixing apples and oranges" when Romney, seizing on a point made earlier by Texas Gov. Rick Perry, slammed Cain's 9-9-9 plan as adding a tax burden on Nevadans or anyone else who already pays a sales tax. That state tax has to be paid anyway, Cain explained, regardless of what happens at the federal level. Romney seems to have gotten the better of Cain in that exchange, with his joke about how Americans would have a new tax -- and a bushel of both apples and oranges. But the idea that Cain pushed back against Romney's characterization probably also helped reinforce the accusation that opponents direct at Romney -- that he misleads.

-- Perry accused Romney of engaging in "hypocrisy" for huffing and puffing on the need to control the border and fight illegal immigration when, according to a December 2006 story in the Boston Globe, Romney hired a lawn company that employed illegal immigrants from Guatemala to work at his home in Belmont, Massachusetts. A year later, in December 2007, the Globe published a follow-up story that reported that Romney continued to employ the company. He eventually fired the company. Frustrated by Romney's insistence that he had done nothing wrong, Perry scolded: "It's time for you to tell the truth, Mitt." Perry has also just released a new ad attacking Romney's veracity; the title of the ad: "Mitt-leading."

-- Newt Gingrich tore into Romney for suggesting he had gotten the idea of an individual mandate to buy health insurance from Gingrich. "That is not true," the former House speaker said in a stern voice. The idea actually came from the Heritage Foundation, a conservative think tank in Washington. Gingrich forced Romney to walk back his comments, and the former college professor seemed to relish his role in teaching Romney a lesson: Don't exaggerate the facts. Gingrich supported the idea of a mandate, but it wasn't -- as Romney claimed -- his idea originally. Gingrich forced Romney to walk back his comments, and the former college professor seemed to relish his role in teaching Romney a lesson: Don't exaggerate the facts. He even tweaked Romney a little, commending his revised version of events with: "That's a little broader. Keep going."

Add it all up and you'll see that some of Romney's fellow presidential hopefuls are frustrated and fed up with his tendency to shade the truth. Most politicians do that, of course, but not all of them are equally good at it. Romney excels at it. What he's not so good at is smoothly shifting from one position to another on controversial issues such as abortion or legalizing undocumented immigrants.

Romney's inconsistencies grate on his opponents. They see him up-close, on stage and in their face. Look at the Las Vegas debate again and read the body language. It's clear that Romney's opponents would not buy a used car from this man.

While some in the media cling to the narrative that Romney "looks presidential," the message sent by some of his opponents this week was simple and direct: buyer beware.

To some of the practitioners in his trade, Romney is the worst kind of politician, the kind who will say anything and pretend to be anything to get elected. Remember, it's the view of many Republicans, from conservative radio talk show hosts to members of the tea party, that the current occupant of the White House is untruthful and inauthentic. Plus, many of these people probably don't see the point in replacing Obama with more of the same.

That would go a long way toward explaining why Romney -- despite substantial resources, a first-rate campaign organization, head-start in running for president, and the experience of having done it before -- isn't performing better in the polls. He can't seem to get much beyond 25 percent in most surveys, with three-fourths of Republicans opting for the ABR camp: "Anybody But Romney."

If Romney ultimately turns out to be the Republican nominee, before he can even begin to convince the American people that he can be president, he'll first have to convince them that he's authentic and that he can be trusted.

Good luck with that, Mitt.

Editor's note: Ruben Navarrette Jr. is a CNN.com contributor and a nationally syndicated columnist. The opinions expressed in this commentary are solely those of Ruben Navarrette Jr.

™ & © 2011 Cable News Network, Inc., a Time Warner Company. All rights reserved.