A persuasive caveat comes from the (so far as I know) impartial J-Ger:

Most of the losing presidential candidates in the past cycles have had issues with “authenticity”, i.e., voters’ suspicions that they’re trying to sell themselves as something they’re not. Budget hawk and Washington insider Bob Dole ran as a tax-cutting conservative (who was hip to what the kids were into) who was going to clean up Washington in the corrupt Clinton era. Al Gore, the Ecology Professor Who Talks To You Slowly Because He Knows You’re Stupid, tried to sell himself as a man of the people, secretly fun and easygoing, passionate about Tipper (the convention kiss)…. the senator’s son who grew up in a hotel suite was going to represent the people, not the powerful. John Kerry was Thurston Howell III, and he was supposed to be a hunter, a simultaneous war hero and war protester, etc.
 
If people think Romney is really a golly-gee CEO tech geek who pragmatically changes tactics and policy stances when they don’t work, trying to pass himself off as the embodied blend of the social conservatism of Gary Bauer with the uncompromising intractability of Jack Bauer, his campaign won’t work.  

Geraghty writes this in the context of a two-man GOP showdown involving Mayor Giuliani.  I think it has much more applicability when the other guy is Fred Thompson.

However, Romney still leads big in Iowa and New Hampshire, so the “authenticity” factor hasn’t impacted Mitt yet.

UPDATE: Dean Barnett, who isn’t remotely objective, offers an equally as persuasive a reason why:

In this little clip, Barney Frank (appearing on my familiar stomping grounds, the Jim Braude Show on New England Cable News) dismisses Romney as “the most intellectually dishonest human being in the history of politics.” (No mention of where he ranks among intellectually dishonest gods or animals.)

Barney’s variety of hysterical hyperbole, a calling-card for wild-eyed liberalism, has served Romney well as he’s gotten to meet the voters. Romney’s opponents, undone by their typical leftwing hysteria, have unwittingly set the bar very low for him. When voters see him, they walk away impressed, and amazed at how what they’ve seen doesn’t match the hostile rhetoric that his opponents have hurled at him. 

DB suggests that “Fred-heads” are setting their man up for a fall by building up expectations about his candidacy to unsustainable levels.  Speaking as one whose preference has narrowed to (You know which) two, I wouldn’t sweat it.

It brings to my mind the story about the two natives who were desperately trying to get away from a pursuing hungry tiger.  Suddenly one of them stops to put on his sneakers. 

The other exclaims, “Why are you wasting your time putting on running shoes?  That tiger is faster than either one of us.”

His now fleeter-of-foot companion, resuming his flight, looks back over his shoulder and tosses off, “I don’t have to be faster than the tiger; I just have to be faster than you.”

If the GOP field wasn’t mediocre, Fred Thompson wouldn’t be in this race.  He doesn’t have to beat himself to win it.

But that’s probably the only way he can lose.