Other candidates' weaknesses are so obvious that they can be quickly summarized. Ron Paul looks and sounds zany.
Yes, his attacks on the Federal Reserve are more plausible than they were four years ago, and he's on his way to doubling his percentage in New Hampshire, as he did in Iowa. But he's not going to attract more votes with a brochure cluttered with arcane verbiage and with keywords in ALL CAPS.
As for Rick Perry, all you have to do is watch that agonizing 53 second brain freeze again. Perry's weakness is that he's never had a secret ambition to be president of the United States.
He got into the race when other politicians' decisions not to run seemed to create an opening for the governor of the nation's second largest and best job-creating state. But his sketchy knowledge of national and foreign issues revealed him as a man who had already achieved his life's ambition as governor of Texas.
Newt Gingrich's weaknesses? Where do we start? On the stump in Iowa, he was constantly detouring off message, and his pledge not to campaign negatively was at odds with the attack dog tactics that enabled him to end Democrats' 40-year majority in the House.
But his greatest weakness, I think, is that he sees himself as a world historical figure. Being derailed in Iowa by negative ads pointing out the $1.6 million he pocketed from Freddie Mac seems to him as enraging as if Winston Churchill had been shoved aside in May 1940 because of his (genuinely) dodgy finances. But rage doesn't attract votes.
Which leaves Mitt Romney, well ahead in New Hampshire polls, ahead in post-Christmas polls in South Carolina, with the resources and organization needed to win in giant Florida.
His weakness is that he never experienced the cultural revolution of the 1960s and so sounds corny and insincere. So far, that hasn't been disabling.